diff --git a/cmd/lambda/lambda.go b/cmd/lambda/lambda.go
index 2679d27..3d8570f 100644
--- a/cmd/lambda/lambda.go
+++ b/cmd/lambda/lambda.go
@@ -5,7 +5,6 @@ import (
 
 	"git.maximhutz.com/max/lambda/internal/cli"
 	"git.maximhutz.com/max/lambda/internal/config"
-	"git.maximhutz.com/max/lambda/internal/engine"
 	"git.maximhutz.com/max/lambda/internal/plugins"
 	"git.maximhutz.com/max/lambda/pkg/convert"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
@@ -34,36 +33,35 @@ func main() {
 	compiled := convert.SaccharineToLambda(ast)
 	logger.Info("compiled λ expression", "tree", compiled.String())
 
-	// Create reduction engine with normal order reducer.
+	// Create reducer.
 	reducer := lambda.NewNormalOrderReducer()
-	process := engine.New(options, compiled, reducer)
 
-	// If the user selected to track CPU performance, attach a profiler to the
-	// process.
+	// If the user selected to track CPU performance, attach a profiler.
 	if options.Profile != "" {
-		plugins.NewPerformance(options.Profile, process)
+		plugins.NewPerformance(options.Profile, reducer)
 	}
 
 	// If the user selected to produce a step-by-step explanation, attach an
-	// observer here.
+	// observer.
 	if options.Explanation {
-		plugins.NewExplanation(process)
+		plugins.NewExplanation(reducer)
 	}
 
-	// If the user opted to track statistics, attach a tracker here, too.
+	// If the user opted to track statistics, attach a tracker.
 	if options.Statistics {
-		plugins.NewStatistics(process)
+		plugins.NewStatistics(reducer)
 	}
 
 	// If the user selected for verbose debug logs, attach a reduction tracker.
 	if options.Verbose {
-		plugins.NewLogs(logger, process)
+		plugins.NewLogs(logger, reducer)
 	}
 
-	process.Run()
+	// Run reduction.
+	reducer.Reduce(compiled)
 
 	// Return the final reduced result.
-	result := process.Expression.String()
+	result := reducer.Expression().String()
 	err = options.Destination.Write(result)
 	cli.HandleError(err)
 }
diff --git a/cmd/lambda/lambda_test.go b/cmd/lambda/lambda_test.go
index 6c84fd6..3855fc5 100644
--- a/cmd/lambda/lambda_test.go
+++ b/cmd/lambda/lambda_test.go
@@ -6,8 +6,6 @@ import (
 	"strings"
 	"testing"
 
-	"git.maximhutz.com/max/lambda/internal/config"
-	"git.maximhutz.com/max/lambda/internal/engine"
 	"git.maximhutz.com/max/lambda/pkg/convert"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
@@ -31,22 +29,11 @@ func runSample(samplePath string) (string, error) {
 	// Compile expression to lambda calculus.
 	compiled := convert.SaccharineToLambda(ast)
 
-	// Create minimal config for benchmarking.
-	cfg := &config.Config{
-		Source:      config.StringSource{Data: ""},
-		Destination: config.StdoutDestination{},
-		Profile:     "",
-		Explanation: false,
-		Statistics:  false,
-		Verbose:     false,
-	}
-
-	// Create and run the engine with normal order reducer.
+	// Create and run the reducer.
 	reducer := lambda.NewNormalOrderReducer()
-	process := engine.New(cfg, compiled, reducer)
-	process.Run()
+	reducer.Reduce(compiled)
 
-	return process.Expression.String() + "\n", nil
+	return reducer.Expression().String() + "\n", nil
 }
 
 // Test that all samples produce expected output.
diff --git a/internal/engine/engine.go b/internal/engine/engine.go
deleted file mode 100644
index ea075b4..0000000
--- a/internal/engine/engine.go
+++ /dev/null
@@ -1,40 +0,0 @@
-// Package "engine" provides an extensible interface for users to interact with
-// λ-calculus and other expression evaluation modes.
-package engine
-
-import (
-	"git.maximhutz.com/max/lambda/internal/config"
-	"git.maximhutz.com/max/lambda/pkg/emitter"
-	"git.maximhutz.com/max/lambda/pkg/expr"
-	"git.maximhutz.com/max/lambda/pkg/reducer"
-)
-
-// Engine is a process for reducing one expression.
-type Engine struct {
-	Config     *config.Config
-	Expression expr.Expression
-	Reducer    reducer.Reducer
-	emitter.BaseEmitter[Event]
-}
-
-// New creates a new engine with the given expression and reducer.
-func New(config *config.Config, expression expr.Expression, reducer reducer.Reducer) *Engine {
-	return &Engine{
-		Config:      config,
-		Expression:  expression,
-		Reducer:     reducer,
-		BaseEmitter: *emitter.New[Event](),
-	}
-}
-
-// Run begins the reduction process.
-func (e *Engine) Run() {
-	e.Emit(StartEvent)
-
-	e.Expression = e.Reducer.Reduce(e.Expression, func(reduced expr.Expression) {
-		e.Expression = reduced
-		e.Emit(StepEvent)
-	})
-
-	e.Emit(StopEvent)
-}
diff --git a/internal/engine/events.go b/internal/engine/events.go
deleted file mode 100644
index 995ebea..0000000
--- a/internal/engine/events.go
+++ /dev/null
@@ -1,9 +0,0 @@
-package engine
-
-type Event int
-
-const (
-	StartEvent Event = iota
-	StepEvent
-	StopEvent
-)
diff --git a/internal/plugins/debug.go b/internal/plugins/debug.go
index 1c1a95b..62fbffd 100644
--- a/internal/plugins/debug.go
+++ b/internal/plugins/debug.go
@@ -3,21 +3,21 @@ package plugins
 import (
 	"log/slog"
 
-	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/pkg/reducer"
 )
 
 type Logs struct {
 	logger  *slog.Logger
-	process *engine.Engine
+	reducer reducer.Reducer
 }
 
-func NewLogs(logger *slog.Logger, process *engine.Engine) *Logs {
-	plugin := &Logs{logger, process}
-	process.On(engine.StepEvent, plugin.Step)
+func NewLogs(logger *slog.Logger, r reducer.Reducer) *Logs {
+	plugin := &Logs{logger, r}
+	r.On(reducer.StepEvent, plugin.Step)
 
 	return plugin
 }
 
 func (t *Logs) Step() {
-	t.logger.Info("reduction", "tree", t.process.Expression.String())
+	t.logger.Info("reduction", "tree", t.reducer.Expression().String())
 }
diff --git a/internal/plugins/explanation.go b/internal/plugins/explanation.go
index ef46bb9..08285c2 100644
--- a/internal/plugins/explanation.go
+++ b/internal/plugins/explanation.go
@@ -5,27 +5,27 @@ package plugins
 import (
 	"fmt"
 
-	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/pkg/reducer"
 )
 
 // Track the reductions made by a reduction process.
 type Explanation struct {
-	process *engine.Engine
+	reducer reducer.Reducer
 }
 
-// Attaches a new explanation tracker to a process.
-func NewExplanation(process *engine.Engine) *Explanation {
-	plugin := &Explanation{process: process}
-	process.On(engine.StartEvent, plugin.Start)
-	process.On(engine.StepEvent, plugin.Step)
+// Attaches a new explanation tracker to a reducer.
+func NewExplanation(r reducer.Reducer) *Explanation {
+	plugin := &Explanation{reducer: r}
+	r.On(reducer.StartEvent, plugin.Start)
+	r.On(reducer.StepEvent, plugin.Step)
 
 	return plugin
 }
 
 func (t *Explanation) Start() {
-	fmt.Println(t.process.Expression.String())
+	fmt.Println(t.reducer.Expression().String())
 }
 
 func (t *Explanation) Step() {
-	fmt.Println("    =", t.process.Expression.String())
+	fmt.Println("    =", t.reducer.Expression().String())
 }
diff --git a/internal/plugins/performance.go b/internal/plugins/performance.go
index 0dc69d4..08cc46b 100644
--- a/internal/plugins/performance.go
+++ b/internal/plugins/performance.go
@@ -1,4 +1,4 @@
-// Package "performance" provides a tracker to observer CPU performance during
+// Package "performance" provides a tracker to observe CPU performance during
 // execution.
 package plugins
 
@@ -7,7 +7,7 @@ import (
 	"path/filepath"
 	"runtime/pprof"
 
-	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/pkg/reducer"
 )
 
 // Observes a reduction process, and publishes a CPU performance profile on
@@ -19,10 +19,10 @@ type Performance struct {
 }
 
 // Create a performance tracker that outputs a profile to "file".
-func NewPerformance(file string, process *engine.Engine) *Performance {
+func NewPerformance(file string, r reducer.Reducer) *Performance {
 	plugin := &Performance{File: file}
-	process.On(engine.StartEvent, plugin.Start)
-	process.On(engine.StopEvent, plugin.Stop)
+	r.On(reducer.StartEvent, plugin.Start)
+	r.On(reducer.StopEvent, plugin.Stop)
 
 	return plugin
 }
diff --git a/internal/plugins/statistics.go b/internal/plugins/statistics.go
index 62223e4..b54ab62 100644
--- a/internal/plugins/statistics.go
+++ b/internal/plugins/statistics.go
@@ -5,8 +5,8 @@ import (
 	"os"
 	"time"
 
-	"git.maximhutz.com/max/lambda/internal/engine"
 	"git.maximhutz.com/max/lambda/internal/statistics"
+	"git.maximhutz.com/max/lambda/pkg/reducer"
 )
 
 // An observer, to track reduction performance.
@@ -16,11 +16,11 @@ type Statistics struct {
 }
 
 // Create a new reduction performance Statistics.
-func NewStatistics(process *engine.Engine) *Statistics {
+func NewStatistics(r reducer.Reducer) *Statistics {
 	plugin := &Statistics{}
-	process.On(engine.StartEvent, plugin.Start)
-	process.On(engine.StepEvent, plugin.Step)
-	process.On(engine.StopEvent, plugin.Stop)
+	r.On(reducer.StartEvent, plugin.Start)
+	r.On(reducer.StepEvent, plugin.Step)
+	r.On(reducer.StopEvent, plugin.Stop)
 
 	return plugin
 }
diff --git a/pkg/emitter/emitter.go b/pkg/emitter/emitter.go
index 594a18a..d387fba 100644
--- a/pkg/emitter/emitter.go
+++ b/pkg/emitter/emitter.go
@@ -3,7 +3,7 @@ package emitter
 import "git.maximhutz.com/max/lambda/pkg/set"
 
 type Emitter[E comparable] interface {
-	On(string, func()) Listener[E]
+	On(E, func()) Listener[E]
 	Off(Listener[E])
 	Emit(E)
 }
@@ -22,6 +22,13 @@ func (e *BaseEmitter[E]) On(kind E, fn func()) Listener[E] {
 	return listener
 }
 
+func (e *BaseEmitter[E]) Off(listener Listener[E]) {
+	kind := listener.Kind()
+	if e.listeners[kind] != nil {
+		e.listeners[kind].Remove(listener)
+	}
+}
+
 func (e *BaseEmitter[E]) Emit(event E) {
 	if e.listeners[event] == nil {
 		e.listeners[event] = set.New[Listener[E]]()
diff --git a/pkg/lambda/reducer.go b/pkg/lambda/reducer.go
index b806c62..2fe0d80 100644
--- a/pkg/lambda/reducer.go
+++ b/pkg/lambda/reducer.go
@@ -1,6 +1,7 @@
 package lambda
 
 import (
+	"git.maximhutz.com/max/lambda/pkg/emitter"
 	"git.maximhutz.com/max/lambda/pkg/expr"
 	"git.maximhutz.com/max/lambda/pkg/reducer"
 )
@@ -10,24 +11,42 @@ var _ reducer.Reducer = (*NormalOrderReducer)(nil)
 
 // NormalOrderReducer implements normal order (leftmost-outermost) reduction
 // for lambda calculus expressions.
-type NormalOrderReducer struct{}
+type NormalOrderReducer struct {
+	emitter.BaseEmitter[reducer.Event]
+	expression expr.Expression
+}
 
 // NewNormalOrderReducer creates a new normal order reducer.
 func NewNormalOrderReducer() *NormalOrderReducer {
-	return &NormalOrderReducer{}
+	return &NormalOrderReducer{
+		BaseEmitter: *emitter.New[reducer.Event](),
+	}
+}
+
+// Expression returns the current expression state.
+func (r *NormalOrderReducer) Expression() expr.Expression {
+	return r.expression
 }
 
 // Reduce performs normal order reduction on a lambda expression.
 // The expression must be a lambda.Expression; other types are returned unchanged.
-func (r *NormalOrderReducer) Reduce(e expr.Expression, onStep func(expr.Expression)) expr.Expression {
+func (r *NormalOrderReducer) Reduce(e expr.Expression) expr.Expression {
+	r.expression = e
+
 	lambdaExpr, ok := e.(Expression)
 	if !ok {
 		return e
 	}
 
+	r.Emit(reducer.StartEvent)
+
 	ReduceAll(&lambdaExpr, func() {
-		onStep(lambdaExpr)
+		r.expression = lambdaExpr
+		r.Emit(reducer.StepEvent)
 	})
 
+	r.expression = lambdaExpr
+	r.Emit(reducer.StopEvent)
+
 	return lambdaExpr
 }
diff --git a/pkg/reducer/events.go b/pkg/reducer/events.go
new file mode 100644
index 0000000..ad432fe
--- /dev/null
+++ b/pkg/reducer/events.go
@@ -0,0 +1,13 @@
+package reducer
+
+// Event represents lifecycle events during reduction.
+type Event int
+
+const (
+	// StartEvent is emitted before reduction begins.
+	StartEvent Event = iota
+	// StepEvent is emitted after each reduction step.
+	StepEvent
+	// StopEvent is emitted after reduction completes.
+	StopEvent
+)
diff --git a/pkg/reducer/reducer.go b/pkg/reducer/reducer.go
index 75d2dd9..ec2d02a 100644
--- a/pkg/reducer/reducer.go
+++ b/pkg/reducer/reducer.go
@@ -2,14 +2,26 @@
 // reduction strategies.
 package reducer
 
-import "git.maximhutz.com/max/lambda/pkg/expr"
+import (
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+	"git.maximhutz.com/max/lambda/pkg/expr"
+)
 
 // Reducer defines the interface for expression reduction strategies.
 // Different evaluation modes (normal order, applicative order, SKI combinators,
 // etc.) implement this interface with their own reduction logic.
+//
+// Reducers also implement the Emitter interface to allow plugins to observe
+// reduction lifecycle events (Start, Step, Stop).
 type Reducer interface {
-	// Reduce performs all reduction steps on the expression, calling onStep
-	// after each reduction.
+	emitter.Emitter[Event]
+
+	// Reduce performs all reduction steps on the expression.
+	// Emits StartEvent before reduction, StepEvent after each step, and
+	// StopEvent after completion.
 	// Returns the final reduced expression.
-	Reduce(e expr.Expression, onStep func(expr.Expression)) expr.Expression
+	Reduce(e expr.Expression) expr.Expression
+
+	// Expression returns the current expression state.
+	Expression() expr.Expression
 }