diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index e9866f3..1fcc816 100644
--- a/evaluator/evaluator.go
+++ b/evaluator/evaluator.go
@@ -19,29 +19,39 @@ func isError(obj object.Object) bool {
 	return false
 }
 
-func Eval(node ast.Node) object.Object {
+func Eval(node ast.Node, env *object.Environment) object.Object {
 	switch node := node.(type) {
 
 	// Statements
 	case *ast.Program:
-		return evalProgram(node)
+		return evalProgram(node, env)
 
 	case *ast.ExpressionStatement:
-		return Eval(node.Expression)
+		return Eval(node.Expression, env)
 
 	case *ast.BlockStatement:
-		return evalBlockStatements(node)
+		return evalBlockStatements(node, env)
 
 	case *ast.IfExpression:
-		return evalIfExpression(node)
+		return evalIfExpression(node, env)
 
 	case *ast.ReturnStatement:
-		val := Eval(node.ReturnValue)
+		val := Eval(node.ReturnValue, env)
 		if isError(val) {
 			return val
 		}
 		return &object.ReturnValue{Value: val}
 
+	case *ast.LetStatement:
+		val := Eval(node.Value, env)
+		if isError(val) {
+			return val
+		}
+		env.Set(node.Name.Value, val)
+
+	case *ast.Identifier:
+		return evalIdentifier(node, env)
+
 	// Expressions
 	case *ast.IntegerLiteral:
 		return &object.Integer{Value: node.Value}
@@ -50,18 +60,18 @@ func Eval(node ast.Node) object.Object {
 		return nativeBoolToBooleanObject(node.Value)
 
 	case *ast.PrefixExpression:
-		right := Eval(node.Right)
+		right := Eval(node.Right, env)
 		if isError(right) {
 			return right
 		}
 		return evalPrefixExpression(node.Operator, right)
 
 	case *ast.InfixExpression:
-		left := Eval(node.Left)
+		left := Eval(node.Left, env)
 		if isError(left) {
 			return left
 		}
-		right := Eval(node.Right)
+		right := Eval(node.Right, env)
 		if isError(right) {
 			return right
 		}
@@ -72,11 +82,11 @@ func Eval(node ast.Node) object.Object {
 	return nil
 }
 
-func evalProgram(program *ast.Program) object.Object {
+func evalProgram(program *ast.Program, env *object.Environment) object.Object {
 	var result object.Object
 
 	for _, statement := range program.Statements {
-		result = Eval(statement)
+		result = Eval(statement, env)
 
 		switch result := result.(type) {
 		case *object.ReturnValue:
@@ -90,11 +100,11 @@ func evalProgram(program *ast.Program) object.Object {
 	return result
 }
 
-func evalBlockStatements(block *ast.BlockStatement) object.Object {
+func evalBlockStatements(block *ast.BlockStatement, env *object.Environment) object.Object {
 	var result object.Object
 
 	for _, statement := range block.Statements {
-		result = Eval(statement)
+		result = Eval(statement, env)
 
 		if result != nil {
 			rt := result.Type()
@@ -188,16 +198,16 @@ func evalIntegerInfixExpression(operator string, left, right object.Object) obje
 	}
 }
 
-func evalIfExpression(ie *ast.IfExpression) object.Object {
-	condition := Eval(ie.Condition)
+func evalIfExpression(ie *ast.IfExpression, env *object.Environment) object.Object {
+	condition := Eval(ie.Condition, env)
 	if isError(condition) {
 		return condition
 	}
 
 	if isTruthy(condition) {
-		return Eval(ie.Consequence)
+		return Eval(ie.Consequence, env)
 	} else if ie.Alternative != nil {
-		return Eval(ie.Alternative)
+		return Eval(ie.Alternative, env)
 	} else {
 		return NULL
 	}
@@ -219,3 +229,11 @@ func isTruthy(obj object.Object) bool {
 func newError(format string, a ...interface{}) *object.Error {
 	return &object.Error{Message: fmt.Sprintf(format, a...)}
 }
+
+func evalIdentifier(node *ast.Identifier, env *object.Environment) object.Object {
+	val, ok := env.Get(node.Value)
+	if !ok {
+		return newError("identifier not found: " + node.Value)
+	}
+	return val
+}
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index 335753c..569c32c 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -180,6 +180,10 @@ func TestErrorHandling(t *testing.T) {
 				`,
 			"unknown operator: BOOLEAN + BOOLEAN",
 		},
+		{
+			"foobar",
+			"identifier not found: foobar",
+		},
 	}
 
 	for _, tt := range tests {
@@ -199,12 +203,29 @@ func TestErrorHandling(t *testing.T) {
 	}
 }
 
+func TestLetStatements(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected int64
+	}{
+		{"let a = 5; a;", 5},
+		{"let a = 5 * 5; a;", 25},
+		{"let a = 5; let b = a; b;", 5},
+		{"let a = 5; let b = a; let c = a + b + 5; c;", 15},
+	}
+
+	for _, tt := range tests {
+		testIntegerObject(t, testEval(tt.input), tt.expected)
+	}
+}
+
 func testEval(input string) object.Object {
 	l := lexer.New(input)
 	p := parser.New(l)
 	program := p.ParseProgram()
+	env := object.NewEnvironment()
 
-	return Eval(program)
+	return Eval(program, env)
 }
 
 func testIntegerObject(t *testing.T, obj object.Object, expected int64) bool {
diff --git a/object/environment.go b/object/environment.go
new file mode 100644
index 0000000..bd89056
--- /dev/null
+++ b/object/environment.go
@@ -0,0 +1,20 @@
+package object
+
+func NewEnvironment() *Environment {
+	s := make(map[string]Object)
+	return &Environment{store: s}
+}
+
+type Environment struct {
+	store map[string]Object
+}
+
+func (e *Environment) Get(name string) (Object, bool) {
+	obj, ok := e.store[name]
+	return obj, ok
+}
+
+func (e *Environment) Set(name string, val Object) Object {
+	e.store[name] = val
+	return val
+}
diff --git a/repl/repl.go b/repl/repl.go
index 94d10e7..bb3df33 100644
--- a/repl/repl.go
+++ b/repl/repl.go
@@ -6,6 +6,7 @@ import (
 	"io"
 	"monkey/evaluator"
 	"monkey/lexer"
+	"monkey/object"
 	"monkey/parser"
 )
 
@@ -13,6 +14,7 @@ const PROMPT = ">> "
 
 func Start(in io.Reader, out io.Writer) {
 	scanner := bufio.NewScanner(in)
+	env := object.NewEnvironment()
 
 	for {
 		fmt.Fprintf(out, PROMPT)
@@ -31,7 +33,7 @@ func Start(in io.Reader, out io.Writer) {
 			continue
 		}
 
-		evaluated := evaluator.Eval(program)
+		evaluated := evaluator.Eval(program, env)
 		if evaluated != nil {
 			io.WriteString(out, evaluated.Inspect())
 			io.WriteString(out, "\n")