diff --git a/model/labelname.go b/model/labelname.go
index c019df4fa..3b4d3a626 100644
--- a/model/labelname.go
+++ b/model/labelname.go
@@ -22,6 +22,12 @@ const (
 	InstanceLabel = LabelName("instance")
 	// The metric name for the synthetic health variable.
 	ScrapeHealthMetricName = LabelValue("up")
+	// The metric name for synthetic alert timeseries.
+	AlertMetricName = LabelValue("ALERTS")
+	// The label name indicating the name of an alert.
+	AlertNameLabel = LabelName("alertname")
+	// The label name indicating the state of an alert.
+	AlertStateLabel = LabelName("alertstate")
 )
 
 // A LabelName is a key for a LabelSet or Metric.  It has a value associated
diff --git a/model/metric.go b/model/metric.go
index 8db9d7802..191fa3233 100644
--- a/model/metric.go
+++ b/model/metric.go
@@ -67,6 +67,14 @@ func (l LabelSet) String() string {
 	return buffer.String()
 }
 
+func (l LabelSet) ToMetric() (metric Metric) {
+	metric = Metric{}
+	for label, value := range l {
+		metric[label] = value
+	}
+	return
+}
+
 // A Metric is similar to a LabelSet, but the key difference is that a Metric is
 // a singleton and refers to one and only one stream of samples.
 type Metric map[LabelName]LabelValue
diff --git a/rules/alerting.go b/rules/alerting.go
new file mode 100644
index 000000000..b26c4900e
--- /dev/null
+++ b/rules/alerting.go
@@ -0,0 +1,143 @@
+// Copyright 2013 Prometheus Team
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package rules
+
+import (
+	"github.com/prometheus/prometheus/model"
+	"github.com/prometheus/prometheus/rules/ast"
+	"github.com/prometheus/prometheus/utility"
+	"time"
+)
+
+// States that active alerts can be in.
+type alertState int
+
+func (s alertState) String() (state string) {
+	switch s {
+	case PENDING:
+		state = "pending"
+	case FIRING:
+		state = "firing"
+	}
+	return
+}
+
+const (
+	PENDING alertState = iota
+	FIRING
+)
+
+// alert is used to track active (pending/firing) alerts over time.
+type alert struct {
+	// The name of the alert.
+	name        string
+	// The vector element labelset triggering this alert.
+	metric      model.Metric
+	// The state of the alert (PENDING or FIRING).
+	state       alertState
+	// The time when the alert first transitioned into PENDING state.
+	activeSince time.Time
+}
+
+// sample returns a Sample suitable for recording the alert.
+func (a alert) sample(timestamp time.Time, value model.SampleValue) model.Sample {
+	recordedMetric := model.Metric{}
+	for label, value := range a.metric {
+		recordedMetric[label] = value
+	}
+
+	recordedMetric[model.MetricNameLabel] = model.AlertMetricName
+	recordedMetric[model.AlertNameLabel] = model.LabelValue(a.name)
+	recordedMetric[model.AlertStateLabel] = model.LabelValue(a.state.String())
+
+	return model.Sample{
+		Metric:    recordedMetric,
+		Value:     value,
+		Timestamp: timestamp,
+	}
+}
+
+// An alerting rule generates alerts from its vector expression.
+type AlertingRule struct {
+	// The name of the alert.
+	name         string
+  // The vector expression from which to generate alerts.
+	vector       ast.VectorNode
+	// The duration for which a labelset needs to persist in the expression
+	// output vector before an alert transitions from PENDING to FIRING state.
+	holdDuration time.Duration
+	// Extra labels to attach to the resulting alert sample vectors.
+	labels       model.LabelSet
+	// A map of alerts which are currently active (PENDING or FIRING), keyed by
+	// the fingerprint of the labelset they correspond to.
+	activeAlerts map[model.Fingerprint]*alert
+}
+
+func (rule AlertingRule) Name() string { return rule.name }
+
+func (rule AlertingRule) EvalRaw(timestamp time.Time) (vector ast.Vector, err error) {
+	return ast.EvalVectorInstant(rule.vector, timestamp)
+}
+
+func (rule AlertingRule) Eval(timestamp time.Time) (vector ast.Vector, err error) {
+	// Get the raw value of the rule expression.
+	exprResult, err := rule.EvalRaw(timestamp)
+	if err != nil {
+		return
+	}
+
+	// Create pending alerts for any new vector elements in the alert expression.
+	resultFingerprints := utility.Set{}
+	for _, sample := range exprResult {
+		fp := model.NewFingerprintFromMetric(sample.Metric)
+		resultFingerprints.Add(fp)
+
+		if _, ok := rule.activeAlerts[fp]; !ok {
+			rule.activeAlerts[fp] = &alert{
+				name:        rule.name,
+				metric:      sample.Metric,
+				state:       PENDING,
+				activeSince: timestamp,
+			}
+		}
+	}
+
+	// Check if any pending alerts should be removed or fire now. Write out alert timeseries.
+	for fp, activeAlert := range rule.activeAlerts {
+		if !resultFingerprints.Has(fp) {
+			vector = append(vector, activeAlert.sample(timestamp, 0))
+			delete(rule.activeAlerts, fp)
+			continue
+		}
+
+		if activeAlert.state == PENDING && timestamp.Sub(activeAlert.activeSince) >= rule.holdDuration {
+			vector = append(vector, activeAlert.sample(timestamp, 0))
+			activeAlert.state = FIRING
+		}
+
+		vector = append(vector, activeAlert.sample(timestamp, 1))
+	}
+	return
+}
+
+// Construct a new AlertingRule.
+func NewAlertingRule(name string, vector ast.VectorNode, holdDuration time.Duration, labels model.LabelSet) *AlertingRule {
+	return &AlertingRule{
+		name:         name,
+		vector:       vector,
+		holdDuration: holdDuration,
+		labels:       labels,
+		activeAlerts: map[model.Fingerprint]*alert{},
+	}
+}
diff --git a/rules/manager.go b/rules/manager.go
index ba86019ec..d9bcda390 100644
--- a/rules/manager.go
+++ b/rules/manager.go
@@ -72,7 +72,7 @@ func (m *ruleManager) runIteration(results chan *Result) {
 	for _, rule := range m.rules {
 		wg.Add(1)
 		go func(rule Rule) {
-			vector, err := rule.Eval(&now)
+			vector, err := rule.Eval(now)
 			m.results <- &Result{
 				Samples: vector,
 				Err:     err,
diff --git a/rules/recording.go b/rules/recording.go
new file mode 100644
index 000000000..8dbe1782b
--- /dev/null
+++ b/rules/recording.go
@@ -0,0 +1,76 @@
+// Copyright 2013 Prometheus Team
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package rules
+
+import (
+	"fmt"
+	"github.com/prometheus/prometheus/model"
+	"github.com/prometheus/prometheus/rules/ast"
+	"time"
+)
+
+// A RecordingRule records its vector expression into new timeseries.
+type RecordingRule struct {
+	name      string
+	vector    ast.VectorNode
+	labels    model.LabelSet
+	permanent bool
+}
+
+func (rule RecordingRule) Name() string { return rule.name }
+
+func (rule RecordingRule) EvalRaw(timestamp time.Time) (vector ast.Vector, err error) {
+	return ast.EvalVectorInstant(rule.vector, timestamp)
+}
+
+func (rule RecordingRule) Eval(timestamp time.Time) (vector ast.Vector, err error) {
+	// Get the raw value of the rule expression.
+	vector, err = rule.EvalRaw(timestamp)
+	if err != nil {
+		return
+	}
+
+	// Override the metric name and labels.
+	for _, sample := range vector {
+		sample.Metric[model.MetricNameLabel] = model.LabelValue(rule.name)
+		for label, value := range rule.labels {
+			if value == "" {
+				delete(sample.Metric, label)
+			} else {
+				sample.Metric[label] = value
+			}
+		}
+	}
+	return
+}
+
+// RuleToDotGraph returns a Graphviz dot graph of the recording rule.
+func (rule RecordingRule) RuleToDotGraph() string {
+	graph := "digraph \"Rules\" {\n"
+	graph += fmt.Sprintf("%#p[shape=\"box\",label=\"%v = \"];\n", rule, rule.name)
+	graph += fmt.Sprintf("%#p -> %#p;\n", &rule, rule.vector)
+	graph += rule.vector.NodeTreeToDotGraph()
+	graph += "}\n"
+	return graph
+}
+
+// Construct a new RecordingRule.
+func NewRecordingRule(name string, labels model.LabelSet, vector ast.VectorNode, permanent bool) *RecordingRule {
+	return &RecordingRule{
+		name:      name,
+		labels:    labels,
+		vector:    vector,
+		permanent: permanent,
+	}
+}
diff --git a/rules/rules.go b/rules/rules.go
index 1f18b5dc0..7a02b3dec 100644
--- a/rules/rules.go
+++ b/rules/rules.go
@@ -14,8 +14,6 @@
 package rules
 
 import (
-	"fmt"
-	"github.com/prometheus/prometheus/model"
 	"github.com/prometheus/prometheus/rules/ast"
 	"time"
 )
@@ -27,94 +25,7 @@ type Rule interface {
 	Name() string
 	// EvalRaw evaluates the rule's vector expression without triggering any
 	// other actions, like recording or alerting.
-	EvalRaw(timestamp *time.Time) (vector ast.Vector, err error)
+	EvalRaw(timestamp time.Time) (vector ast.Vector, err error)
 	// Eval evaluates the rule, including any associated recording or alerting actions.
-	Eval(timestamp *time.Time) (vector ast.Vector, err error)
-}
-
-// A RecordingRule records its vector expression into new timeseries.
-type RecordingRule struct {
-	name      string
-	vector    ast.VectorNode
-	labels    model.LabelSet
-	permanent bool
-}
-
-// An alerting rule generates alerts from its vector expression.
-type AlertingRule struct {
-	name         string
-	vector       ast.VectorNode
-	holdDuration time.Duration
-	labels       model.LabelSet
-}
-
-func (rule RecordingRule) Name() string { return rule.name }
-
-func (rule RecordingRule) EvalRaw(timestamp *time.Time) (vector ast.Vector, err error) {
-	return ast.EvalVectorInstant(rule.vector, *timestamp)
-}
-
-func (rule RecordingRule) Eval(timestamp *time.Time) (vector ast.Vector, err error) {
-	// Get the raw value of the rule expression.
-	vector, err = rule.EvalRaw(timestamp)
-	if err != nil {
-		return
-	}
-
-	// Override the metric name and labels.
-	for _, sample := range vector {
-		sample.Metric[model.MetricNameLabel] = model.LabelValue(rule.name)
-		for label, value := range rule.labels {
-			if value == "" {
-				delete(sample.Metric, label)
-			} else {
-				sample.Metric[label] = value
-			}
-		}
-	}
-	return
-}
-
-func (rule RecordingRule) RuleToDotGraph() string {
-	graph := "digraph \"Rules\" {\n"
-	graph += fmt.Sprintf("%#p[shape=\"box\",label=\"%v = \"];\n", rule, rule.name)
-	graph += fmt.Sprintf("%#p -> %#p;\n", &rule, rule.vector)
-	graph += rule.vector.NodeTreeToDotGraph()
-	graph += "}\n"
-	return graph
-}
-
-func (rule AlertingRule) Name() string { return rule.name }
-
-func (rule AlertingRule) EvalRaw(timestamp *time.Time) (vector ast.Vector, err error) {
-	return ast.EvalVectorInstant(rule.vector, *timestamp)
-}
-
-func (rule AlertingRule) Eval(timestamp *time.Time) (vector ast.Vector, err error) {
-	// Get the raw value of the rule expression.
-	vector, err = rule.EvalRaw(timestamp)
-	if err != nil {
-		return
-	}
-
-	// TODO(julius): handle alerting.
-	return
-}
-
-func NewRecordingRule(name string, labels model.LabelSet, vector ast.VectorNode, permanent bool) *RecordingRule {
-	return &RecordingRule{
-		name:      name,
-		labels:    labels,
-		vector:    vector,
-		permanent: permanent,
-	}
-}
-
-func NewAlertingRule(name string, vector ast.VectorNode, holdDuration time.Duration, labels model.LabelSet) *AlertingRule {
-	return &AlertingRule{
-		name:         name,
-		vector:       vector,
-		holdDuration: holdDuration,
-		labels:       labels,
-	}
+	Eval(timestamp time.Time) (vector ast.Vector, err error)
 }
diff --git a/rules/rules_test.go b/rules/rules_test.go
index 48645e44c..1e13641cb 100644
--- a/rules/rules_test.go
+++ b/rules/rules_test.go
@@ -15,6 +15,7 @@ package rules
 
 import (
 	"fmt"
+	"github.com/prometheus/prometheus/model"
 	"github.com/prometheus/prometheus/rules/ast"
 	"github.com/prometheus/prometheus/storage/metric"
 	"github.com/prometheus/prometheus/utility/test"
@@ -25,242 +26,14 @@ import (
 )
 
 var (
-	testEvalTime = testStartTime.Add(testDuration5m * 10)
+	testEvalTime = testStartTime.Add(testSampleInterval * 10)
 	fixturesPath = "fixtures"
 )
 
-// Labels in expected output need to be alphabetically sorted.
-var expressionTests = []struct {
-	expr           string
-	output         []string
-	shouldFail     bool
-	checkOrder     bool
-	fullRanges     int
-	intervalRanges int
-}{
-	{
-		expr:           "SUM(http_requests)",
-		output:         []string{"http_requests{} => 3600 @[%v]"},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job)",
-		output: []string{
-			"http_requests{job='api-server'} => 1000 @[%v]",
-			"http_requests{job='app-server'} => 2600 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job, group)",
-		output: []string{
-			"http_requests{group='canary',job='api-server'} => 700 @[%v]",
-			"http_requests{group='canary',job='app-server'} => 1500 @[%v]",
-			"http_requests{group='production',job='api-server'} => 300 @[%v]",
-			"http_requests{group='production',job='app-server'} => 1100 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "AVG(http_requests) BY (job)",
-		output: []string{
-			"http_requests{job='api-server'} => 250 @[%v]",
-			"http_requests{job='app-server'} => 650 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "MIN(http_requests) BY (job)",
-		output: []string{
-			"http_requests{job='api-server'} => 100 @[%v]",
-			"http_requests{job='app-server'} => 500 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "MAX(http_requests) BY (job)",
-		output: []string{
-			"http_requests{job='api-server'} => 400 @[%v]",
-			"http_requests{job='app-server'} => 800 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job) - count(http_requests)",
-		output: []string{
-			"http_requests{job='api-server'} => 992 @[%v]",
-			"http_requests{job='app-server'} => 2592 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job) - 2",
-		output: []string{
-			"http_requests{job='api-server'} => 998 @[%v]",
-			"http_requests{job='app-server'} => 2598 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job) % 3",
-		output: []string{
-			"http_requests{job='api-server'} => 1 @[%v]",
-			"http_requests{job='app-server'} => 2 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job) / 0",
-		output: []string{
-			"http_requests{job='api-server'} => +Inf @[%v]",
-			"http_requests{job='app-server'} => +Inf @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job) > 1000",
-		output: []string{
-			"http_requests{job='app-server'} => 2600 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job) <= 1000",
-		output: []string{
-			"http_requests{job='api-server'} => 1000 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job) != 1000",
-		output: []string{
-			"http_requests{job='app-server'} => 2600 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job) == 1000",
-		output: []string{
-			"http_requests{job='api-server'} => 1000 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "SUM(http_requests) BY (job) + SUM(http_requests) BY (job)",
-		output: []string{
-			"http_requests{job='api-server'} => 2000 @[%v]",
-			"http_requests{job='app-server'} => 5200 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "http_requests{job='api-server', group='canary'}",
-		output: []string{
-			"http_requests{group='canary',instance='0',job='api-server'} => 300 @[%v]",
-			"http_requests{group='canary',instance='1',job='api-server'} => 400 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 2,
-	}, {
-		expr: "http_requests{job='api-server', group='canary'} + delta(http_requests{job='api-server'}[5m], 1)",
-		output: []string{
-			"http_requests{group='canary',instance='0',job='api-server'} => 330 @[%v]",
-			"http_requests{group='canary',instance='1',job='api-server'} => 440 @[%v]",
-		},
-		fullRanges:     4,
-		intervalRanges: 0,
-	}, {
-		expr: "delta(http_requests[25m], 1)",
-		output: []string{
-			"http_requests{group='canary',instance='0',job='api-server'} => 150 @[%v]",
-			"http_requests{group='canary',instance='0',job='app-server'} => 350 @[%v]",
-			"http_requests{group='canary',instance='1',job='api-server'} => 200 @[%v]",
-			"http_requests{group='canary',instance='1',job='app-server'} => 400 @[%v]",
-			"http_requests{group='production',instance='0',job='api-server'} => 50 @[%v]",
-			"http_requests{group='production',instance='0',job='app-server'} => 250 @[%v]",
-			"http_requests{group='production',instance='1',job='api-server'} => 100 @[%v]",
-			"http_requests{group='production',instance='1',job='app-server'} => 300 @[%v]",
-		},
-		fullRanges:     8,
-		intervalRanges: 0,
-	}, {
-		expr: "sort(http_requests)",
-		output: []string{
-			"http_requests{group='production',instance='0',job='api-server'} => 100 @[%v]",
-			"http_requests{group='production',instance='1',job='api-server'} => 200 @[%v]",
-			"http_requests{group='canary',instance='0',job='api-server'} => 300 @[%v]",
-			"http_requests{group='canary',instance='1',job='api-server'} => 400 @[%v]",
-			"http_requests{group='production',instance='0',job='app-server'} => 500 @[%v]",
-			"http_requests{group='production',instance='1',job='app-server'} => 600 @[%v]",
-			"http_requests{group='canary',instance='0',job='app-server'} => 700 @[%v]",
-			"http_requests{group='canary',instance='1',job='app-server'} => 800 @[%v]",
-		},
-		checkOrder:     true,
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		expr: "sort_desc(http_requests)",
-		output: []string{
-			"http_requests{group='canary',instance='1',job='app-server'} => 800 @[%v]",
-			"http_requests{group='canary',instance='0',job='app-server'} => 700 @[%v]",
-			"http_requests{group='production',instance='1',job='app-server'} => 600 @[%v]",
-			"http_requests{group='production',instance='0',job='app-server'} => 500 @[%v]",
-			"http_requests{group='canary',instance='1',job='api-server'} => 400 @[%v]",
-			"http_requests{group='canary',instance='0',job='api-server'} => 300 @[%v]",
-			"http_requests{group='production',instance='1',job='api-server'} => 200 @[%v]",
-			"http_requests{group='production',instance='0',job='api-server'} => 100 @[%v]",
-		},
-		checkOrder:     true,
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		// Single-letter label names and values.
-		expr: "x{y='testvalue'}",
-		output: []string{
-			"x{y='testvalue'} => 100 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 1,
-	}, {
-		// Lower-cased aggregation operators should work too.
-		expr: "sum(http_requests) by (job) + min(http_requests) by (job) + max(http_requests) by (job) + avg(http_requests) by (job)",
-		output: []string{
-			"http_requests{job='app-server'} => 4550 @[%v]",
-			"http_requests{job='api-server'} => 1750 @[%v]",
-		},
-		fullRanges:     0,
-		intervalRanges: 8,
-	}, {
-		// Deltas should be adjusted for target interval vs. samples under target interval.
-		expr:           "delta(http_requests{group='canary',instance='1',job='app-server'}[18m], 1)",
-		output:         []string{"http_requests{group='canary',instance='1',job='app-server'} => 288 @[%v]"},
-		fullRanges:     1,
-		intervalRanges: 0,
-	}, {
-		// Rates should transform per-interval deltas to per-second rates.
-		expr:           "rate(http_requests{group='canary',instance='1',job='app-server'}[10m])",
-		output:         []string{"http_requests{group='canary',instance='1',job='app-server'} => 0.26666666666666666 @[%v]"},
-		fullRanges:     1,
-		intervalRanges: 0,
-	}, {
-		// Empty expressions shouldn't parse.
-		expr:       "",
-		shouldFail: true,
-	}, {
-		// Subtracting a vector from a scalar is not supported.
-		expr:       "1 - http_requests",
-		shouldFail: true,
-	}, {
-		// Interval durations can't be in quotes.
-		expr:       "http_requests['1m']",
-		shouldFail: true,
-	},
-}
-
-func annotateWithTime(lines []string) []string {
+func annotateWithTime(lines []string, timestamp time.Time) []string {
 	annotatedLines := []string{}
 	for _, line := range lines {
-		annotatedLines = append(annotatedLines, fmt.Sprintf(line, testEvalTime))
+		annotatedLines = append(annotatedLines, fmt.Sprintf(line, timestamp))
 	}
 	return annotatedLines
 }
@@ -276,22 +49,248 @@ func vectorComparisonString(expected []string, actual []string) string {
 		separator)
 }
 
-func TestExpressions(t *testing.T) {
-	temporaryDirectory := test.NewTemporaryDirectory("rule_expression_tests", t)
-	defer temporaryDirectory.Close()
-	tieredStorage, err := metric.NewTieredStorage(5000, 5000, 100, time.Second*30, time.Second*1, time.Second*20, temporaryDirectory.Path())
-	if err != nil {
-		t.Fatalf("Error opening storage: %s", err)
+func newTestStorage(t test.Tester) (storage metric.Storage, closer test.Closer) {
+	storage, closer = metric.NewTestTieredStorage(t)
+	ast.SetStorage(storage)
+	storeMatrix(storage, testMatrix)
+	return
+}
+
+func ExpressionTests(t *testing.T) {
+	// Labels in expected output need to be alphabetically sorted.
+	var expressionTests = []struct {
+		expr           string
+		output         []string
+		shouldFail     bool
+		checkOrder     bool
+		fullRanges     int
+		intervalRanges int
+	}{
+		{
+			expr:           "SUM(http_requests)",
+			output:         []string{"http_requests{} => 3600 @[%v]"},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job)",
+			output: []string{
+				"http_requests{job='api-server'} => 1000 @[%v]",
+				"http_requests{job='app-server'} => 2600 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job, group)",
+			output: []string{
+				"http_requests{group='canary',job='api-server'} => 700 @[%v]",
+				"http_requests{group='canary',job='app-server'} => 1500 @[%v]",
+				"http_requests{group='production',job='api-server'} => 300 @[%v]",
+				"http_requests{group='production',job='app-server'} => 1100 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "AVG(http_requests) BY (job)",
+			output: []string{
+				"http_requests{job='api-server'} => 250 @[%v]",
+				"http_requests{job='app-server'} => 650 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "MIN(http_requests) BY (job)",
+			output: []string{
+				"http_requests{job='api-server'} => 100 @[%v]",
+				"http_requests{job='app-server'} => 500 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "MAX(http_requests) BY (job)",
+			output: []string{
+				"http_requests{job='api-server'} => 400 @[%v]",
+				"http_requests{job='app-server'} => 800 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job) - count(http_requests)",
+			output: []string{
+				"http_requests{job='api-server'} => 992 @[%v]",
+				"http_requests{job='app-server'} => 2592 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job) - 2",
+			output: []string{
+				"http_requests{job='api-server'} => 998 @[%v]",
+				"http_requests{job='app-server'} => 2598 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job) % 3",
+			output: []string{
+				"http_requests{job='api-server'} => 1 @[%v]",
+				"http_requests{job='app-server'} => 2 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job) / 0",
+			output: []string{
+				"http_requests{job='api-server'} => +Inf @[%v]",
+				"http_requests{job='app-server'} => +Inf @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job) > 1000",
+			output: []string{
+				"http_requests{job='app-server'} => 2600 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job) <= 1000",
+			output: []string{
+				"http_requests{job='api-server'} => 1000 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job) != 1000",
+			output: []string{
+				"http_requests{job='app-server'} => 2600 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job) == 1000",
+			output: []string{
+				"http_requests{job='api-server'} => 1000 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "SUM(http_requests) BY (job) + SUM(http_requests) BY (job)",
+			output: []string{
+				"http_requests{job='api-server'} => 2000 @[%v]",
+				"http_requests{job='app-server'} => 5200 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "http_requests{job='api-server', group='canary'}",
+			output: []string{
+				"http_requests{group='canary',instance='0',job='api-server'} => 300 @[%v]",
+				"http_requests{group='canary',instance='1',job='api-server'} => 400 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 2,
+		}, {
+			expr: "http_requests{job='api-server', group='canary'} + delta(http_requests{job='api-server'}[5m], 1)",
+			output: []string{
+				"http_requests{group='canary',instance='0',job='api-server'} => 330 @[%v]",
+				"http_requests{group='canary',instance='1',job='api-server'} => 440 @[%v]",
+			},
+			fullRanges:     4,
+			intervalRanges: 0,
+		}, {
+			expr: "delta(http_requests[25m], 1)",
+			output: []string{
+				"http_requests{group='canary',instance='0',job='api-server'} => 150 @[%v]",
+				"http_requests{group='canary',instance='0',job='app-server'} => 350 @[%v]",
+				"http_requests{group='canary',instance='1',job='api-server'} => 200 @[%v]",
+				"http_requests{group='canary',instance='1',job='app-server'} => 400 @[%v]",
+				"http_requests{group='production',instance='0',job='api-server'} => 50 @[%v]",
+				"http_requests{group='production',instance='0',job='app-server'} => 250 @[%v]",
+				"http_requests{group='production',instance='1',job='api-server'} => 100 @[%v]",
+				"http_requests{group='production',instance='1',job='app-server'} => 300 @[%v]",
+			},
+			fullRanges:     8,
+			intervalRanges: 0,
+		}, {
+			expr: "sort(http_requests)",
+			output: []string{
+				"http_requests{group='production',instance='0',job='api-server'} => 100 @[%v]",
+				"http_requests{group='production',instance='1',job='api-server'} => 200 @[%v]",
+				"http_requests{group='canary',instance='0',job='api-server'} => 300 @[%v]",
+				"http_requests{group='canary',instance='1',job='api-server'} => 400 @[%v]",
+				"http_requests{group='production',instance='0',job='app-server'} => 500 @[%v]",
+				"http_requests{group='production',instance='1',job='app-server'} => 600 @[%v]",
+				"http_requests{group='canary',instance='0',job='app-server'} => 700 @[%v]",
+				"http_requests{group='canary',instance='1',job='app-server'} => 800 @[%v]",
+			},
+			checkOrder:     true,
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			expr: "sort_desc(http_requests)",
+			output: []string{
+				"http_requests{group='canary',instance='1',job='app-server'} => 800 @[%v]",
+				"http_requests{group='canary',instance='0',job='app-server'} => 700 @[%v]",
+				"http_requests{group='production',instance='1',job='app-server'} => 600 @[%v]",
+				"http_requests{group='production',instance='0',job='app-server'} => 500 @[%v]",
+				"http_requests{group='canary',instance='1',job='api-server'} => 400 @[%v]",
+				"http_requests{group='canary',instance='0',job='api-server'} => 300 @[%v]",
+				"http_requests{group='production',instance='1',job='api-server'} => 200 @[%v]",
+				"http_requests{group='production',instance='0',job='api-server'} => 100 @[%v]",
+			},
+			checkOrder:     true,
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			// Single-letter label names and values.
+			expr: "x{y='testvalue'}",
+			output: []string{
+				"x{y='testvalue'} => 100 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 1,
+		}, {
+			// Lower-cased aggregation operators should work too.
+			expr: "sum(http_requests) by (job) + min(http_requests) by (job) + max(http_requests) by (job) + avg(http_requests) by (job)",
+			output: []string{
+				"http_requests{job='app-server'} => 4550 @[%v]",
+				"http_requests{job='api-server'} => 1750 @[%v]",
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			// Deltas should be adjusted for target interval vs. samples under target interval.
+			expr:           "delta(http_requests{group='canary',instance='1',job='app-server'}[18m], 1)",
+			output:         []string{"http_requests{group='canary',instance='1',job='app-server'} => 288 @[%v]"},
+			fullRanges:     1,
+			intervalRanges: 0,
+		}, {
+			// Rates should transform per-interval deltas to per-second rates.
+			expr:           "rate(http_requests{group='canary',instance='1',job='app-server'}[10m])",
+			output:         []string{"http_requests{group='canary',instance='1',job='app-server'} => 0.26666666666666666 @[%v]"},
+			fullRanges:     1,
+			intervalRanges: 0,
+		}, {
+			// Empty expressions shouldn't parse.
+			expr:       "",
+			shouldFail: true,
+		}, {
+			// Subtracting a vector from a scalar is not supported.
+			expr:       "1 - http_requests",
+			shouldFail: true,
+		}, {
+			// Interval durations can't be in quotes.
+			expr:       "http_requests['1m']",
+			shouldFail: true,
+		},
 	}
-	go tieredStorage.Serve()
 
-	ast.SetStorage(tieredStorage)
-
-	storeMatrix(tieredStorage, testMatrix)
+	tieredStorage, closer := newTestStorage(t)
+	defer closer.Close()
 	tieredStorage.Flush()
 
 	for i, exprTest := range expressionTests {
-		expectedLines := annotateWithTime(exprTest.output)
+		expectedLines := annotateWithTime(exprTest.output, testEvalTime)
 
 		testExpr, err := LoadExprFromString(exprTest.expr)
 
@@ -299,25 +298,25 @@ func TestExpressions(t *testing.T) {
 			if exprTest.shouldFail {
 				continue
 			}
-			t.Errorf("%d Error during parsing: %v", i, err)
-			t.Errorf("%d Expression: %v", i, exprTest.expr)
+			t.Errorf("%d. Error during parsing: %v", i, err)
+			t.Errorf("%d. Expression: %v", i, exprTest.expr)
 		} else {
 			if exprTest.shouldFail {
-				t.Errorf("%d Test should fail, but didn't", i)
+				t.Errorf("%d. Test should fail, but didn't", i)
 			}
 			failed := false
 			resultStr := ast.EvalToString(testExpr, testEvalTime, ast.TEXT)
 			resultLines := strings.Split(resultStr, "\n")
 
 			if len(exprTest.output) != len(resultLines) {
-				t.Errorf("%d Number of samples in expected and actual output don't match", i)
+				t.Errorf("%d. Number of samples in expected and actual output don't match", i)
 				failed = true
 			}
 
 			if exprTest.checkOrder {
 				for j, expectedSample := range expectedLines {
 					if resultLines[j] != expectedSample {
-						t.Errorf("%d.%d Expected sample '%v', got '%v'", i, j, resultLines[j], expectedSample)
+						t.Errorf("%d.%d. Expected sample '%v', got '%v'", i, j, resultLines[j], expectedSample)
 						failed = true
 					}
 				}
@@ -330,7 +329,7 @@ func TestExpressions(t *testing.T) {
 						}
 					}
 					if !found {
-						t.Errorf("%d.%d Couldn't find expected sample in output: '%v'", i, j, expectedSample)
+						t.Errorf("%d.%d. Couldn't find expected sample in output: '%v'", i, j, expectedSample)
 						failed = true
 					}
 				}
@@ -339,16 +338,16 @@ func TestExpressions(t *testing.T) {
 			analyzer := ast.NewQueryAnalyzer()
 			analyzer.AnalyzeQueries(testExpr)
 			if exprTest.fullRanges != len(analyzer.FullRanges) {
-				t.Errorf("%d Count of full ranges didn't match: %v vs %v", i, exprTest.fullRanges, len(analyzer.FullRanges))
+				t.Errorf("%d. Count of full ranges didn't match: %v vs %v", i, exprTest.fullRanges, len(analyzer.FullRanges))
 				failed = true
 			}
 			if exprTest.intervalRanges != len(analyzer.IntervalRanges) {
-				t.Errorf("%d Count of interval ranges didn't match: %v vs %v", i, exprTest.intervalRanges, len(analyzer.IntervalRanges))
+				t.Errorf("%d. Count of interval ranges didn't match: %v vs %v", i, exprTest.intervalRanges, len(analyzer.IntervalRanges))
 				failed = true
 			}
 
 			if failed {
-				t.Errorf("%d Expression: %v\n%v", i, exprTest.expr, vectorComparisonString(expectedLines, resultLines))
+				t.Errorf("%d. Expression: %v\n%v", i, exprTest.expr, vectorComparisonString(expectedLines, resultLines))
 			}
 		}
 	}
@@ -420,3 +419,79 @@ func TestRules(t *testing.T) {
 		}
 	}
 }
+
+func TestAlertingRule(t *testing.T) {
+	// Labels in expected output need to be alphabetically sorted.
+	var evalOutputs = [][]string{
+		{
+			"ALERTS{alertname='HttpRequestRateLow',alertstate='pending',group='canary',instance='0',job='app-server'} => 1 @[%v]",
+			"ALERTS{alertname='HttpRequestRateLow',alertstate='pending',group='canary',instance='1',job='app-server'} => 1 @[%v]",
+		},
+		{
+			"ALERTS{alertname='HttpRequestRateLow',alertstate='pending',group='canary',instance='0',job='app-server'} => 0 @[%v]",
+			"ALERTS{alertname='HttpRequestRateLow',alertstate='firing',group='canary',instance='0',job='app-server'} => 1 @[%v]",
+			"ALERTS{alertname='HttpRequestRateLow',alertstate='pending',group='canary',instance='1',job='app-server'} => 0 @[%v]",
+			"ALERTS{alertname='HttpRequestRateLow',alertstate='firing',group='canary',instance='1',job='app-server'} => 1 @[%v]",
+		},
+		{
+			"ALERTS{alertname='HttpRequestRateLow',alertstate='firing',group='canary',instance='1',job='app-server'} => 0 @[%v]",
+			"ALERTS{alertname='HttpRequestRateLow',alertstate='firing',group='canary',instance='0',job='app-server'} => 0 @[%v]",
+		},
+		{
+		/* empty */
+		},
+		{
+		/* empty */
+		},
+	}
+
+	tieredStorage, closer := newTestStorage(t)
+	defer closer.Close()
+	tieredStorage.Flush()
+
+	alertExpr, err := LoadExprFromString("http_requests{group='canary',job='app-server'} < 100")
+	if err != nil {
+		t.Fatalf("Unable to parse alert expression: %s", err)
+	}
+	alertName := "HttpRequestRateLow"
+	alertLabels := model.LabelSet{
+		"summary": "HTTP request rate is low",
+	}
+	rule := NewAlertingRule(alertName, alertExpr.(ast.VectorNode), time.Minute, alertLabels)
+
+	for i, expected := range evalOutputs {
+		evalTime := testStartTime.Add(testSampleInterval * time.Duration(i))
+		actual, err := rule.Eval(evalTime)
+		if err != nil {
+			t.Fatalf("Error during alerting rule evaluation: %s", err)
+		}
+		actualLines := strings.Split(actual.String(), "\n")
+		expectedLines := annotateWithTime(expected, evalTime)
+		if actualLines[0] == "" {
+			actualLines = []string{}
+		}
+
+		failed := false
+		if len(actualLines) != len(expectedLines) {
+			t.Errorf("%d. Number of samples in expected and actual output don't match (%d vs. %d)", i, len(expectedLines), len(actualLines))
+			failed = true
+		}
+
+		for j, expectedSample := range expectedLines {
+			found := false
+			for _, actualSample := range actualLines {
+				if actualSample == expectedSample {
+					found = true
+				}
+			}
+			if !found {
+				t.Errorf("%d.%d. Couldn't find expected sample in output: '%v'", i, j, expectedSample)
+				failed = true
+			}
+		}
+
+		if failed {
+			t.Fatalf("%d. Expected and actual outputs don't match:\n%v", i, vectorComparisonString(expectedLines, actualLines))
+		}
+	}
+}
diff --git a/rules/testdata.go b/rules/testdata.go
index e14234aa3..ae449ba63 100644
--- a/rules/testdata.go
+++ b/rules/testdata.go
@@ -20,7 +20,7 @@ import (
 	"time"
 )
 
-var testDuration5m = time.Duration(5) * time.Minute
+var testSampleInterval = time.Duration(5) * time.Minute
 var testStartTime = time.Time{}
 
 func getTestValueStream(startVal model.SampleValue,
@@ -33,7 +33,7 @@ func getTestValueStream(startVal model.SampleValue,
 			Timestamp: currentTime,
 		}
 		resultValues = append(resultValues, sample)
-		currentTime = currentTime.Add(testDuration5m)
+		currentTime = currentTime.Add(testSampleInterval)
 	}
 	return resultValues
 }
diff --git a/storage/metric/leveldb.go b/storage/metric/leveldb.go
index 653827735..ee8255916 100644
--- a/storage/metric/leveldb.go
+++ b/storage/metric/leveldb.go
@@ -75,7 +75,9 @@ func (l *LevelDBMetricPersistence) Close() {
 	for _, closer := range persistences {
 		closerGroup.Add(1)
 		go func(closer leveldbCloser) {
-			closer.Close()
+			if closer != nil {
+				closer.Close()
+			}
 			closerGroup.Done()
 		}(closer)
 	}
diff --git a/storage/metric/test_helper.go b/storage/metric/test_helper.go
index a63c6ac3b..26d9bd8d8 100644
--- a/storage/metric/test_helper.go
+++ b/storage/metric/test_helper.go
@@ -72,3 +72,39 @@ func buildMemoryTestPersistence(f func(p MetricPersistence, t test.Tester)) func
 		f(p, t)
 	}
 }
+
+type testTieredStorageCloser struct {
+	storage   Storage
+	directory test.Closer
+}
+
+func (t testTieredStorageCloser) Close() {
+	t.storage.Close()
+	t.directory.Close()
+}
+
+func NewTestTieredStorage(t test.Tester) (storage Storage, closer test.Closer) {
+	var directory test.TemporaryDirectory
+	directory = test.NewTemporaryDirectory("test_tiered_storage", t)
+	storage, err := NewTieredStorage(5000000, 2500, 1000, 5*time.Second, 15*time.Second, 0*time.Second, directory.Path())
+
+	if err != nil {
+		if storage != nil {
+			storage.Close()
+		}
+		directory.Close()
+		t.Fatalf("Error creating storage: %s", err)
+	}
+
+	if storage == nil {
+		directory.Close()
+		t.Fatalf("storage == nil")
+	}
+
+	go storage.Serve()
+	closer = &testTieredStorageCloser{
+		storage:   storage,
+		directory: directory,
+	}
+	return
+}
diff --git a/storage/metric/tiered_test.go b/storage/metric/tiered_test.go
index ff9138291..77f274364 100644
--- a/storage/metric/tiered_test.go
+++ b/storage/metric/tiered_test.go
@@ -21,37 +21,6 @@ import (
 	"time"
 )
 
-type testTieredStorageCloser struct {
-	storage   Storage
-	directory test.Closer
-}
-
-func (t testTieredStorageCloser) Close() {
-	t.storage.Close()
-	t.directory.Close()
-}
-
-func newTestTieredStorage(t test.Tester) (storage Storage, closer test.Closer) {
-	var directory test.TemporaryDirectory
-	directory = test.NewTemporaryDirectory("test_tiered_storage", t)
-	storage, err := NewTieredStorage(5000000, 2500, 1000, 5*time.Second, 15*time.Second, 0*time.Second, directory.Path())
-
-	if err != nil {
-		t.Fatalf("Error creating storage: %s", err)
-	}
-
-	if storage == nil {
-		t.Fatalf("storage == nil")
-	}
-
-	go storage.Serve()
-	closer = &testTieredStorageCloser{
-		storage:   storage,
-		directory: directory,
-	}
-	return
-}
-
 func buildSamples(from, to time.Time, interval time.Duration, m model.Metric) (v []model.Sample) {
 	i := model.SampleValue(0)
 
@@ -369,7 +338,7 @@ func testMakeView(t test.Tester, flushToDisk bool) {
 	)
 
 	for i, scenario := range scenarios {
-		tiered, closer := newTestTieredStorage(t)
+		tiered, closer := NewTestTieredStorage(t)
 
 		for j, datum := range scenario.data {
 			err := tiered.AppendSample(datum)
@@ -507,7 +476,7 @@ func TestGetAllValuesForLabel(t *testing.T) {
 	}
 
 	for i, scenario := range scenarios {
-		tiered, closer := newTestTieredStorage(t)
+		tiered, closer := NewTestTieredStorage(t)
 		for j, metric := range scenario.in {
 			sample := model.Sample{
 				Metric: model.Metric{model.MetricNameLabel: model.LabelValue(metric.metricName)},
@@ -542,7 +511,7 @@ func TestGetAllValuesForLabel(t *testing.T) {
 }
 
 func TestGetFingerprintsForLabelSet(t *testing.T) {
-	tiered, closer := newTestTieredStorage(t)
+	tiered, closer := NewTestTieredStorage(t)
 	defer closer.Close()
 	memorySample := model.Sample{
 		Metric: model.Metric{model.MetricNameLabel: "http_requests", "method": "/foo"},