diff --git a/ast/ast.go b/ast/ast.go
index 34540b1..fbbbf70 100644
--- a/ast/ast.go
+++ b/ast/ast.go
@@ -1,9 +1,13 @@
 package ast
 
-import "monkey/token"
+import (
+	"bytes"
+	"monkey/token"
+)
 
 type Node interface {
 	TokenLiteral() string
+	String() string
 }
 
 type Statement interface {
@@ -28,6 +32,16 @@ func (p *Program) TokenLiteral() string {
 	}
 }
 
+func (p *Program) String() string {
+	var out bytes.Buffer
+
+	for _, s := range p.Statements {
+		out.WriteString(s.String())
+	}
+
+	return out.String()
+}
+
 type Identifier struct {
 	Token token.Token // the token.Ident token
 	Value string
@@ -44,20 +58,102 @@ type ReturnStatement struct {
 	ReturnValue Expression
 }
 
+type ExpressionStatement struct {
+	Token      token.Token // the first token of the expression
+	Expression Expression
+}
+
+type IntegerLiteral struct {
+	Token token.Token
+	Value int64
+}
+
+type PrefixExpression struct {
+	Token    token.Token
+	Operator string
+	Right    Expression
+}
+
 func (ls *LetStatement) statementNode() {
 }
 func (ls *LetStatement) TokenLiteral() string {
 	return ls.Token.Literal
 }
+func (ls *LetStatement) String() string {
+	var out bytes.Buffer
+
+	out.WriteString(ls.TokenLiteral() + " ")
+	out.WriteString(ls.Name.String())
+	out.WriteString(" = ")
+
+	if ls.Value != nil {
+		out.WriteString(ls.Value.String())
+	}
+
+	out.WriteString(";")
+
+	return out.String()
+}
 
 func (i *Identifier) expressionNode() {
 }
 func (i *Identifier) TokenLiteral() string {
 	return i.Token.Literal
 }
+func (i *Identifier) String() string { return i.Value }
 
-func (i *ReturnStatement) statementNode() {
+func (rs *ReturnStatement) statementNode() {
 }
-func (i *ReturnStatement) TokenLiteral() string {
-	return i.Token.Literal
+func (rs *ReturnStatement) TokenLiteral() string {
+	return rs.Token.Literal
+}
+func (rs *ReturnStatement) String() string {
+	var out bytes.Buffer
+
+	out.WriteString(rs.TokenLiteral() + " ")
+
+	if rs.ReturnValue != nil {
+		out.WriteString(rs.ReturnValue.String())
+	}
+
+	out.WriteString(";")
+
+	return out.String()
+}
+
+func (es *ExpressionStatement) statementNode() {
+}
+func (es *ExpressionStatement) TokenLiteral() string {
+	return es.Token.Literal
+}
+func (es *ExpressionStatement) String() string {
+	if es.Expression != nil {
+		return es.Expression.String()
+	}
+
+	return ""
+}
+
+func (il *IntegerLiteral) expressionNode() {
+}
+func (il *IntegerLiteral) TokenLiteral() string {
+	return il.Token.Literal
+}
+func (il *IntegerLiteral) String() string {
+	return il.Token.Literal
+}
+
+func (pe *PrefixExpression) expressionNode() {}
+func (pe *PrefixExpression) TokenLiteral() string {
+	return pe.Token.Literal
+}
+func (pe *PrefixExpression) String() string {
+	var out bytes.Buffer
+
+	out.WriteString("(")
+	out.WriteString(pe.Operator)
+	out.WriteString(pe.Right.String())
+	out.WriteString(")")
+
+	return out.String()
 }
diff --git a/ast/ast_test.go b/ast/ast_test.go
new file mode 100644
index 0000000..e1bea83
--- /dev/null
+++ b/ast/ast_test.go
@@ -0,0 +1,28 @@
+package ast
+
+import (
+	"monkey/token"
+	"testing"
+)
+
+func TestString(t *testing.T) {
+	program := &Program{
+		Statements: []Statement{
+			&LetStatement{
+				Token: token.Token{Type: token.LET, Literal: "let"},
+				Name: &Identifier{
+					Token: token.Token{Type: token.IDENT, Literal: "myVar"},
+					Value: "myVar",
+				},
+				Value: &Identifier{
+					Token: token.Token{Type: token.IDENT, Literal: "anotherVar"},
+					Value: "anotherVar",
+				},
+			},
+		},
+	}
+
+	if program.String() != "let myVar = anotherVar;" {
+		t.Errorf("program.String wrong. got=%q", program.String())
+	}
+}
diff --git a/parser/parser.go b/parser/parser.go
index ee8e718..a42dd6a 100644
--- a/parser/parser.go
+++ b/parser/parser.go
@@ -5,6 +5,18 @@ import (
 	"monkey/ast"
 	"monkey/lexer"
 	"monkey/token"
+	"strconv"
+)
+
+const (
+	_ int = iota
+	LOWEST
+	EQUALS      // ==
+	LESSGREATER // > or <
+	SUM         // +
+	PRODUCT     // *
+	PREFIX      // -X or !X
+	CALL        // myFunction(X)
 )
 
 type Parser struct {
@@ -13,8 +25,16 @@ type Parser struct {
 
 	curToken  token.Token
 	peekToken token.Token
+
+	prefixParseFns map[token.TokenType]prefixParseFn
+	infixParseFns  map[token.TokenType]infixParseFn
 }
 
+type (
+	prefixParseFn func() ast.Expression
+	infixParseFn  func(expression ast.Expression) ast.Expression
+)
+
 func (p *Parser) nextToken() {
 	p.curToken = p.peekToken
 	p.peekToken = p.l.NextToken()
@@ -26,6 +46,12 @@ func New(l *lexer.Lexer) *Parser {
 		errors: []string{},
 	}
 
+	p.prefixParseFns = make(map[token.TokenType]prefixParseFn)
+	p.registerPrefix(token.IDENT, p.parseIdentifier)
+	p.registerPrefix(token.INT, p.parseIntegerLiteral)
+	p.registerPrefix(token.BANG, p.parsePrefixExpression)
+	p.registerPrefix(token.MINUS, p.parsePrefixExpression)
+
 	// Read two token, so curToken and peekToken are both set
 	p.nextToken()
 	p.nextToken()
@@ -55,7 +81,7 @@ func (p *Parser) parseStatement() ast.Statement {
 	case token.RETURN:
 		return p.parseReturnStatement()
 	default:
-		return nil
+		return p.parseExpressionsStatement()
 	}
 }
 
@@ -123,3 +149,74 @@ func (p *Parser) parseReturnStatement() *ast.ReturnStatement {
 
 	return stmt
 }
+
+func (p *Parser) parseExpressionsStatement() *ast.ExpressionStatement {
+	stmt := &ast.ExpressionStatement{Token: p.curToken}
+
+	stmt.Expression = p.parseExpression(LOWEST)
+
+	if p.peekTokenIs(token.SEMICOLON) {
+		p.nextToken()
+	}
+
+	return stmt
+}
+
+func (p *Parser) parseExpression(precedence int) ast.Expression {
+	prefix := p.prefixParseFns[p.curToken.Type]
+	if prefix == nil {
+		p.noPrefixParseFnError(p.curToken.Type)
+		return nil
+	}
+	leftExp := prefix()
+
+	return leftExp
+}
+
+func (p *Parser) parseIntegerLiteral() ast.Expression {
+	lit := &ast.IntegerLiteral{Token: p.curToken}
+
+	value, err := strconv.ParseInt(p.curToken.Literal, 0, 64)
+	if err != nil {
+		msg := fmt.Sprintf("could not parse %q as integer", p.curToken.Literal)
+		p.errors = append(p.errors, msg)
+		return nil
+	}
+
+	lit.Value = value
+
+	return lit
+}
+
+func (p *Parser) registerPrefix(tokenType token.TokenType, fn prefixParseFn) {
+	p.prefixParseFns[tokenType] = fn
+}
+
+func (p *Parser) registerInfix(tokenType token.TokenType, fn infixParseFn) {
+	p.infixParseFns[tokenType] = fn
+}
+
+func (p *Parser) parseIdentifier() ast.Expression {
+	return &ast.Identifier{
+		Token: p.curToken,
+		Value: p.curToken.Literal,
+	}
+}
+
+func (p *Parser) noPrefixParseFnError(t token.TokenType) {
+	msg := fmt.Sprintf("no prefix parse function for %s found", t)
+	p.errors = append(p.errors, msg)
+}
+
+func (p *Parser) parsePrefixExpression() ast.Expression {
+	expression := &ast.PrefixExpression{
+		Token:    p.curToken,
+		Operator: p.curToken.Literal,
+	}
+
+	p.nextToken()
+
+	expression.Right = p.parseExpression(PREFIX)
+
+	return expression
+}
diff --git a/parser/parser_test.go b/parser/parser_test.go
index f6c317f..69023ad 100644
--- a/parser/parser_test.go
+++ b/parser/parser_test.go
@@ -1,6 +1,7 @@
 package parser
 
 import (
+	"fmt"
 	"monkey/ast"
 	"monkey/lexer"
 	"testing"
@@ -73,3 +74,116 @@ func testLetStatements(t *testing.T, s ast.Statement, name string) bool {
 
 	return true
 }
+
+func TestIdentifierExpressions(t *testing.T) {
+	input := "foobar;"
+
+	l := lexer.New(input)
+	p := New(l)
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	if len(program.Statements) != 1 {
+		t.Fatalf("program has not enough statements. got=%d", len(program.Statements))
+	}
+	stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("program.Statements[0] is not ast.ExpressionStatement. got=%T", program.Statements[0])
+	}
+
+	ident, ok := stmt.Expression.(*ast.Identifier)
+	if !ok {
+		t.Fatalf("exp is not *ast.Identifier. got=%T", stmt.Expression)
+	}
+	if ident.Value != "foobar" {
+		t.Errorf("ident.Value not %s. got=%s", "foobar", ident.Value)
+	}
+	if ident.TokenLiteral() != "foobar" {
+		t.Errorf("ident.TokenLiteral not %s. got=%s", "foobar", ident.TokenLiteral())
+	}
+}
+
+func TestIntegerLiteralExpressions(t *testing.T) {
+	input := "5;"
+
+	l := lexer.New(input)
+	p := New(l)
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	if len(program.Statements) != 1 {
+		t.Fatalf("program has not enough statements. got=%d", len(program.Statements))
+	}
+	stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("program.Statements[0] is not ast.ExpressionStatement. got=%T", program.Statements[0])
+	}
+
+	literal, ok := stmt.Expression.(*ast.IntegerLiteral)
+	if !ok {
+		t.Fatalf("exp is not *ast.IntegerLiteral. got=%T", stmt.Expression)
+	}
+	if literal.Value != 5 {
+		t.Errorf("ident.Value not %d. got=%d", 5, literal.Value)
+	}
+	if literal.TokenLiteral() != "5" {
+		t.Errorf("ident.TokenLiteral not %s. got=%s", "5", literal.TokenLiteral())
+	}
+}
+
+func TestParsingPrefixExpressions(t *testing.T) {
+	prefixTests := []struct {
+		input        string
+		operator     string
+		integerValue int64
+	}{
+		{"!5;", "!", 5},
+		{"-15;", "-", 15},
+	}
+
+	for _, tt := range prefixTests {
+		l := lexer.New(tt.input)
+		p := New(l)
+		program := p.ParseProgram()
+		checkParserErrors(t, p)
+
+		if len(program.Statements) != 1 {
+			t.Fatalf("program.Statements does not contain %d statements. got=%d\n", 1, len(program.Statements))
+		}
+		stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+		if !ok {
+			t.Fatalf("program.Statements[0] is not ast.ExpressionStatement. got=%T", program.Statements[0])
+		}
+
+		exp, ok := stmt.Expression.(*ast.PrefixExpression)
+		if !ok {
+			t.Fatalf("exp is not *ast.PrefixExpression. got=%T", stmt.Expression)
+		}
+		if exp.Operator != tt.operator {
+			t.Fatalf("exp.Operator is not '%s'. got=%s", tt.operator, exp.Operator)
+		}
+		if !testIntegerLiteral(t, exp.Right, tt.integerValue) {
+			return
+		}
+	}
+}
+
+func testIntegerLiteral(t *testing.T, il ast.Expression, value int64) bool {
+	integ, ok := il.(*ast.IntegerLiteral)
+	if !ok {
+		t.Errorf("il not *ast.IntegerLiteral. got=%T", il)
+		return false
+	}
+
+	if integ.Value != value {
+		t.Errorf("integ.Value not %d. got=%d", value, integ.Value)
+		return false
+	}
+
+	if integ.TokenLiteral() != fmt.Sprintf("%d", value) {
+		t.Errorf("integ.TopkenLiteral not %d. got=%s", value, integ.TokenLiteral())
+		return false
+	}
+
+	return true
+}