diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
new file mode 100644
index 0000000..dc480b9
--- /dev/null
+++ b/evaluator/evaluator.go
@@ -0,0 +1,32 @@
+package evaluator
+
+import (
+	"monkey/ast"
+	"monkey/object"
+)
+
+func Eval(node ast.Node) object.Object {
+	switch node := node.(type) {
+  	// Statements
+	case *ast.Program:
+		return eval_statements(node.Statements)
+
+	case *ast.ExpressionStatement:
+		return Eval(node.Expression)
+
+		// Expressions
+	case *ast.IntegerLiteral:
+		return &object.Integer{Value: node.Value}
+	}
+
+	return nil
+}
+
+func eval_statements(statements []ast.Statement) object.Object {
+	var result object.Object
+
+	for _, statement := range statements {
+		result = Eval(statement)
+	}
+	return result
+}
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
new file mode 100644
index 0000000..aceb434
--- /dev/null
+++ b/evaluator/evaluator_test.go
@@ -0,0 +1,45 @@
+package evaluator
+
+import (
+	"monkey/lexer"
+	"monkey/object"
+	"monkey/parser"
+	"testing"
+)
+
+func TestEvalIntegerExpression(l_test *testing.T) {
+	tests := []struct {
+		input    string
+		expected int64
+	}{
+		{"5", 5},
+		{"10", 10},
+	}
+
+	for _, tt := range tests {
+		evaluated := test_eval(tt.input)
+		test_integer_object(l_test, evaluated, tt.expected)
+	}
+}
+
+// Helpers
+func test_eval(input string) object.Object {
+	l_lexer := lexer.New(input)
+	l_parser := parser.New(l_lexer)
+	program := l_parser.ParseProgram()
+
+	return Eval(program)
+}
+
+func test_integer_object(l_test *testing.T, l_object object.Object, expected int64) bool {
+	result, ok := l_object.(*object.Integer)
+	if !ok {
+		l_test.Errorf("object is not integer, got=%T (%+v)", l_object, l_object)
+		return false
+	}
+	if result.Value != expected {
+		l_test.Errorf("object has wrong value, got=%d, want=%d", result.Value, expected)
+		return false
+	}
+	return true
+}
diff --git a/object/object.go b/object/object.go
new file mode 100644
index 0000000..e81b429
--- /dev/null
+++ b/object/object.go
@@ -0,0 +1,53 @@
+package object
+
+import "fmt"
+
+type ObjectType string
+
+const (
+	INTEGER_OBJECT = "INTEGER"
+	BOOLEAN_OBJECT = "BOOLEAN"
+	NULL_OBJECT    = "NULL"
+)
+
+type Object interface {
+	Type() ObjectType
+	Inspect() string
+}
+
+// Integer
+type Integer struct {
+	Value int64
+}
+
+func (i *Integer) Type() ObjectType {
+	return INTEGER_OBJECT
+}
+
+func (i *Integer) Inspect() string {
+	return fmt.Sprintf("%d", i.Value)
+}
+
+// Booleans
+type Boolean struct {
+	Value bool
+}
+
+func (b *Boolean) Type() ObjectType {
+	return BOOLEAN_OBJECT
+}
+
+func (b *Boolean) Inpect() string {
+	return fmt.Sprintf("%t", b.Value)
+}
+
+// Null
+type Null struct{}
+
+func (n *Null) Type() ObjectType {
+	return NULL_OBJECT
+}
+
+func (n *Null) Inpect() string {
+	return "null"
+}