diff --git a/Architecture.md b/Architecture.md
new file mode 100644
index 0000000..1903bb8
--- /dev/null
+++ b/Architecture.md
@@ -0,0 +1,13 @@
+tt Programming Language Backend Architecture
+
+# Goals
+- Easy support for different architectures and OSs
+- Easily Optimisable on most levels
+- Good Performance
+
+# Architecture
+
+AST --> Type Checking --> TAST --> IR Emission --> TTIR --> Codegen --> TTASM --> Emit --> FASM -> Binary
+
+TTIR: TT Intermediate Representation is the Representation that the AST gets turned into. This will be mostly be used for optimissing and abstracting away from Assembly
+TAST: Typed Ast
diff --git a/ast/ast.go b/ast/ast.go
index d823278..a33dffe 100644
--- a/ast/ast.go
+++ b/ast/ast.go
@@ -63,7 +63,7 @@ type ErrorExpression struct {
 
 func (e *ErrorExpression) expressionNode()      {}
 func (e *ErrorExpression) TokenLiteral() string { return e.InvalidToken.Literal }
-func (e *ErrorExpression) String() string       { return "<ERROR>" }
+func (e *ErrorExpression) String() string       { return "<ERROR EXPR>" }
 
 type IntegerExpression struct {
 	Token token.Token // The token.INT
diff --git a/lexer/lexer.go b/lexer/lexer.go
index a0c6687..8444ecc 100644
--- a/lexer/lexer.go
+++ b/lexer/lexer.go
@@ -64,20 +64,20 @@ func (l *Lexer) NextToken() token.Token {
 
 	switch l.ch {
 	case ';':
-		tok = l.newToken(token.SEMICOLON)
+		tok = l.newToken(token.Semicolon)
 	case '=':
-		tok = l.newToken(token.EQUAL)
+		tok = l.newToken(token.Equal)
 	case '(':
-		tok = l.newToken(token.OPEN_PAREN)
+		tok = l.newToken(token.OpenParen)
 	case ')':
-		tok = l.newToken(token.CLOSE_PAREN)
+		tok = l.newToken(token.CloseParen)
 	case -1:
 		tok.Literal = ""
-		tok.Type = token.EOF
+		tok.Type = token.Eof
 	default:
 		if isNumber(l.ch) {
 			tok.Literal = l.readInteger()
-			tok.Type = token.INT
+			tok.Type = token.Int
 			return tok
 		} else if unicode.IsLetter(l.ch) {
 			tok.Literal = l.readIdentifier()
@@ -87,7 +87,7 @@ func (l *Lexer) NextToken() token.Token {
 			if l.errorCallback != nil {
 				l.errorCallback(tok.Loc, "Unknown character %r", l.ch)
 			}
-			tok = l.newToken(token.ILLEGAL)
+			tok = l.newToken(token.Illegal)
 		}
 	}
 	if err := l.readChar(); err != nil {
@@ -164,7 +164,7 @@ func (l *Lexer) skipWhitespace() {
 
 func (l *Lexer) error(loc token.Loc, format string, args ...any) {
 	if l.errorCallback != nil {
-		l.errorCallback(loc, format, args)
+		l.errorCallback(loc, format, args...)
 	}
 
 	l.errors += 1
diff --git a/lexer/lexer_test.go b/lexer/lexer_test.go
index f4011a8..3f5453c 100644
--- a/lexer/lexer_test.go
+++ b/lexer/lexer_test.go
@@ -17,7 +17,7 @@ func runLexerTest(t *testing.T, test lexerTest) {
 
 	l, err := New(test.input, "test.tt")
 	l.WithErrorCallback(func(l token.Loc, s string, a ...any) {
-		format := fmt.Sprintf(s, a)
+		format := fmt.Sprintf(s, a...)
 		t.Errorf("Lexer error callback called: %s:%d:%d %s", l.File, l.Line, l.Col, format)
 	})
 	if err != nil {
@@ -42,14 +42,14 @@ func TestBasicFunctionality(t *testing.T) {
 	runLexerTest(t, lexerTest{
 		input: "fn main() = 0;",
 		expectedToken: []token.Token{
-			{Type: token.FN, Literal: "fn"},
-			{Type: token.IDENT, Literal: "main"},
-			{Type: token.OPEN_PAREN, Literal: "("},
-			{Type: token.CLOSE_PAREN, Literal: ")"},
-			{Type: token.EQUAL, Literal: "="},
-			{Type: token.INT, Literal: "0"},
-			{Type: token.SEMICOLON, Literal: ";"},
-			{Type: token.EOF, Literal: ""},
+			{Type: token.Fn, Literal: "fn"},
+			{Type: token.Ident, Literal: "main"},
+			{Type: token.OpenParen, Literal: "("},
+			{Type: token.CloseParen, Literal: ")"},
+			{Type: token.Equal, Literal: "="},
+			{Type: token.Int, Literal: "0"},
+			{Type: token.Semicolon, Literal: ";"},
+			{Type: token.Eof, Literal: ""},
 		},
 	})
 }
diff --git a/parser/parser.go b/parser/parser.go
index 901fec7..fe3cb2b 100644
--- a/parser/parser.go
+++ b/parser/parser.go
@@ -1,18 +1,211 @@
 package parser
 
 import (
+	"fmt"
+	"strconv"
+
+	"robaertschi.xyz/robaertschi/tt/ast"
 	"robaertschi.xyz/robaertschi/tt/lexer"
 	"robaertschi.xyz/robaertschi/tt/token"
 )
 
+type precedence int
+
+const (
+	LOWEST precedence = iota
+	SUM
+	PRODUCT
+)
+
 type ErrorCallback func(token.Token, string, ...any)
+type prefixParseFn func() ast.Expression
+type infixParseFn func(ast.Expression) ast.Expression
 
 type Parser struct {
-	lexer lexer.Lexer
-
 	curToken  token.Token
 	peekToken token.Token
 
 	errors        int
 	errorCallback ErrorCallback
+
+	l              *lexer.Lexer
+	prefixParseFns map[token.TokenType]prefixParseFn
+	infixParseFns  map[token.TokenType]infixParseFn
+}
+
+func New(l *lexer.Lexer) *Parser {
+	p := &Parser{l: l}
+
+	p.prefixParseFns = make(map[token.TokenType]prefixParseFn)
+	p.registerPrefixFn(token.Int, p.parseIntegerExpression)
+
+	p.infixParseFns = make(map[token.TokenType]infixParseFn)
+
+	p.nextToken()
+	p.nextToken()
+
+	return p
+}
+
+func (p *Parser) WithErrorCallback(errorCallback ErrorCallback) {
+	p.errorCallback = errorCallback
+}
+
+func (p *Parser) registerInfixFn(tt token.TokenType, infix infixParseFn) {
+	p.infixParseFns[tt] = infix
+}
+
+func (p *Parser) registerPrefixFn(tt token.TokenType, fn prefixParseFn) {
+	p.prefixParseFns[tt] = fn
+}
+
+func (p *Parser) nextToken() {
+	p.curToken = p.peekToken
+	p.peekToken = p.l.NextToken()
+	fmt.Printf("curToken: %q, peekToken: %q\n", p.curToken.Type, p.peekToken.Type)
+}
+
+func (p *Parser) curTokenIs(tt token.TokenType) bool {
+	return p.curToken.Type == tt
+}
+
+func (p *Parser) peekTokenIs(tt token.TokenType) bool {
+	return p.peekToken.Type == tt
+}
+
+func getPrecedence(tt token.TokenType) precedence {
+	switch tt {
+	default:
+		return LOWEST
+	}
+}
+
+func (p *Parser) peekPrecedence() precedence {
+	return getPrecedence(p.peekToken.Type)
+}
+
+func (p *Parser) error(t token.Token, format string, args ...any) {
+	if p.errorCallback != nil {
+		p.errorCallback(t, format, args...)
+	} else {
+		fmt.Printf("%s:%d:%d ", t.Loc.File, t.Loc.Line, t.Loc.Col)
+		fmt.Printf(format, args...)
+		fmt.Println()
+	}
+
+	p.errors += 1
+}
+
+func (p *Parser) exprError(invalidToken token.Token, format string, args ...any) ast.Expression {
+	p.error(invalidToken, format, args...)
+	return &ast.ErrorExpression{
+		InvalidToken: invalidToken,
+	}
+}
+
+func (p *Parser) expect(tt token.TokenType) bool {
+	if p.curToken.Type != tt {
+		p.error(p.curToken, "expected %q, got %q", tt, p.curToken.Type)
+		return false
+	}
+	return true
+}
+
+func (p *Parser) expectPeek(tt token.TokenType) bool {
+	if p.peekToken.Type != tt {
+		p.error(p.peekToken, "expected %q, got %q", tt, p.peekToken.Type)
+		p.nextToken()
+		return false
+	}
+	p.nextToken()
+	return true
+}
+
+func (p *Parser) ParseProgram() *ast.Program {
+	decls := []ast.Declaration{}
+
+	for p.curToken.Type != token.Eof {
+		decl := p.parseDeclaration()
+		if decl != nil {
+			decls = append(decls, decl)
+		}
+		p.nextToken()
+	}
+
+	return &ast.Program{
+		Declarations: decls,
+	}
+}
+
+func (p *Parser) parseDeclaration() ast.Declaration {
+	if !p.expect(token.Fn) {
+		return nil
+	}
+	tok := p.curToken
+	if !p.expectPeek(token.Ident) {
+		return nil
+	}
+
+	name := p.curToken.Literal
+	if !p.expectPeek(token.OpenParen) {
+		return nil
+	}
+	if !p.expectPeek(token.CloseParen) {
+		return nil
+	}
+	if !p.expectPeek(token.Equal) {
+		return nil
+	}
+
+	p.nextToken()
+	expr := p.parseExpression(LOWEST)
+	if !p.expectPeek(token.Semicolon) {
+		return nil
+	}
+
+	return &ast.FunctionDeclaration{
+		Token: tok,
+		Name:  name,
+		Body:  expr,
+	}
+}
+
+func (p *Parser) parseExpression(precedence precedence) ast.Expression {
+	prefix := p.prefixParseFns[p.curToken.Type]
+	if prefix == nil {
+		return p.exprError(p.curToken, "could not parse invalid token in expression %s", p.curToken.Type)
+	}
+
+	leftExpr := prefix()
+
+	for !p.peekTokenIs(token.Semicolon) && precedence < p.peekPrecedence() {
+		infix := p.infixParseFns[p.peekToken.Type]
+		if infix == nil {
+			return leftExpr
+		}
+
+		p.nextToken()
+
+		leftExpr = infix(leftExpr)
+	}
+
+	return leftExpr
+}
+
+func (p *Parser) parseIntegerExpression() ast.Expression {
+	if !p.expect(token.Int) {
+		return &ast.ErrorExpression{InvalidToken: p.curToken}
+	}
+
+	int := &ast.IntegerExpression{
+		Token: p.curToken,
+	}
+
+	value, err := strconv.ParseInt(int.Token.Literal, 0, 64)
+	if err != nil {
+		return p.exprError(int.Token, "invalid integer literal: %v", err)
+	}
+
+	int.Value = value
+	return int
 }
diff --git a/parser/parser_test.go b/parser/parser_test.go
new file mode 100644
index 0000000..4f670a6
--- /dev/null
+++ b/parser/parser_test.go
@@ -0,0 +1,106 @@
+package parser
+
+import (
+	"fmt"
+	"testing"
+
+	"robaertschi.xyz/robaertschi/tt/ast"
+	"robaertschi.xyz/robaertschi/tt/lexer"
+	"robaertschi.xyz/robaertschi/tt/token"
+)
+
+type parserTest struct {
+	input           string
+	expectedProgram ast.Program
+}
+
+func runParserTest(test parserTest, t *testing.T) {
+	t.Helper()
+	l, err := lexer.New(test.input, "test.tt")
+	l.WithErrorCallback(func(l token.Loc, s string, a ...any) {
+		format := fmt.Sprintf(s, a...)
+		t.Errorf("Lexer error callback called: %s:%d:%d %s", l.File, l.Line, l.Col, format)
+	})
+
+	if err != nil {
+		t.Errorf("creating lexer failed: %v", err)
+	}
+
+	p := New(l)
+	p.WithErrorCallback(func(tok token.Token, s string, a ...any) {
+		format := fmt.Sprintf(s, a...)
+		t.Errorf("Parser error callback called: %s:%d:%d %s", tok.Loc.File, tok.Loc.Line, tok.Loc.Col, format)
+	})
+
+	actual := p.ParseProgram()
+
+	if p.errors > 0 {
+		t.Fatalf("parser errors: %d", p.errors)
+	}
+
+	if len(actual.Declarations) != len(test.expectedProgram.Declarations) {
+		t.Fatalf("expected %d declarations, got %d", len(test.expectedProgram.Declarations), len(actual.Declarations))
+	}
+
+	for i, decl := range test.expectedProgram.Declarations {
+		expectDeclarationSame(t, decl, actual.Declarations[i])
+	}
+}
+
+func expectDeclarationSame(t *testing.T, expected ast.Declaration, actual ast.Declaration) {
+	t.Helper()
+
+	switch expected := expected.(type) {
+	case *ast.FunctionDeclaration:
+		actual, ok := actual.(*ast.FunctionDeclaration)
+		if !ok {
+			t.Errorf("expected function declaration, got %T", actual)
+			return
+		}
+		if actual.Name != expected.Name {
+			t.Errorf("expected function name %s, got %s", expected.Name, actual.Name)
+		}
+
+		expectExpression(t, expected.Body, actual.Body)
+	}
+}
+
+func expectExpression(t *testing.T, expected ast.Expression, actual ast.Expression) {
+	t.Helper()
+
+	switch expected := expected.(type) {
+	case *ast.ErrorExpression:
+		actual, ok := actual.(*ast.ErrorExpression)
+		if !ok {
+			t.Errorf("expected error expression, got %T", actual)
+			return
+		}
+		if actual.InvalidToken != expected.InvalidToken {
+			t.Errorf("expected invalid token %v, got %v", expected.InvalidToken, actual.InvalidToken)
+		}
+	case *ast.IntegerExpression:
+		integerExpr, ok := actual.(*ast.IntegerExpression)
+		if !ok {
+			t.Errorf("expected *ast.IntegerExpression, got %T", actual)
+			return
+		}
+		if integerExpr.Value != expected.Value {
+			t.Errorf("expected integer value %d, got %d", expected.Value, integerExpr.Value)
+		}
+	}
+}
+
+func TestFunctionDeclaration(t *testing.T) {
+	test := parserTest{
+		input: "fn main() = 0;",
+		expectedProgram: ast.Program{
+			Declarations: []ast.Declaration{
+				&ast.FunctionDeclaration{
+					Name: "main",
+					Body: &ast.IntegerExpression{Value: 0, Token: token.Token{Type: token.Int, Literal: "0"}},
+				},
+			},
+		},
+	}
+	runParserTest(test, t)
+}
diff --git a/tast/tast.go b/tast/tast.go
new file mode 100644
index 0000000..12e1620
--- /dev/null
+++ b/tast/tast.go
@@ -0,0 +1,76 @@
+// Typed AST
+// Almost identical to the AST, but contains types and contains only correct types.
+// Also, it does not contain a Error Expression, because, that an previous error
+
+package tast
+
+import (
+	"fmt"
+	"strings"
+
+	"robaertschi.xyz/robaertschi/tt/token"
+	"robaertschi.xyz/robaertschi/tt/types"
+)
+
+type Node interface {
+	TokenLiteral() string
+	String() string
+}
+
+type Declaration interface {
+	Node
+	declarationNode()
+}
+
+type Expression interface {
+	Node
+	expressionNode()
+	Type() types.Type
+}
+
+type Program struct {
+	Declarations []Declaration
+}
+
+func (p *Program) TokenLiteral() string {
+	if len(p.Declarations) > 0 {
+		return p.Declarations[0].TokenLiteral()
+	}
+	return ""
+}
+
+func (p *Program) String() string {
+	var builder strings.Builder
+
+	for _, decl := range p.Declarations {
+		builder.WriteString(decl.String())
+		builder.WriteRune('\n')
+	}
+
+	return builder.String()
+}
+
+type FunctionDeclaration struct {
+	Token      token.Token // The token.FN
+	Body       Expression
+	Name       string
+	ReturnType types.Type
+}
+
+func (fd *FunctionDeclaration) declarationNode()     {}
+func (fd *FunctionDeclaration) TokenLiteral() string { return fd.Token.Literal }
+func (fd *FunctionDeclaration) String() string {
+	return fmt.Sprintf("fn %v(): %v = %v;", fd.Name, fd.ReturnType.Name(), fd.Body.String())
+}
+
+type IntegerExpression struct {
+	Token token.Token // The token.INT
+	Value int64
+}
+
+func (ie *IntegerExpression) expressionNode() {}
+func (ie *IntegerExpression) Type() types.Type {
+	return types.I64
+}
+func (ie *IntegerExpression) TokenLiteral() string { return ie.Token.Literal }
+func (ie *IntegerExpression) String() string       { return ie.Token.Literal }
diff --git a/token/token.go b/token/token.go
index 7343987..97862cd 100644
--- a/token/token.go
+++ b/token/token.go
@@ -16,28 +16,28 @@ type Token struct {
 }
 
 var keywords = map[string]TokenType{
-	"fn": FN,
+	"fn": Fn,
 }
 
 const (
-	ILLEGAL TokenType = "ILLEGAL"
-	EOF     TokenType = "EOF"
+	Illegal TokenType = "ILLEGAL"
+	Eof     TokenType = "EOF"
 
-	IDENT TokenType = "IDENT"
-	INT   TokenType = "INT"
+	Ident TokenType = "IDENT"
+	Int   TokenType = "INT"
 
-	SEMICOLON   = ";"
-	EQUAL       = "="
-	OPEN_PAREN  = "("
-	CLOSE_PAREN = ")"
+	Semicolon  = ";"
+	Equal      = "="
+	OpenParen  = "("
+	CloseParen = ")"
 
 	// Keywords
-	FN = "FN"
+	Fn = "FN"
 )
 
 func LookupKeyword(literal string) TokenType {
 	if value, ok := keywords[literal]; ok {
 		return value
 	}
-	return IDENT
+	return Ident
 }
diff --git a/ttir/ttir.go b/ttir/ttir.go
new file mode 100644
index 0000000..5cac9d4
--- /dev/null
+++ b/ttir/ttir.go
@@ -0,0 +1,33 @@
+package ttir
+
+type Program struct {
+	Functions []Function
+}
+
+type Function struct {
+	Name         string
+	Instructions []Instruction
+}
+
+type Instruction interface {
+	String() string
+	instruction()
+}
+
+type Ret struct {
+	op Operand
+}
+
+func (r *Ret) String()      {}
+func (r *Ret) instruction() {}
+
+type Operand interface {
+	String() string
+	operand()
+}
+
+type Constant struct {
+	Value int64
+}
+
+func (c *Constant) operand() {}
diff --git a/typechecker/checker.go b/typechecker/checker.go
new file mode 100644
index 0000000..940bf1f
--- /dev/null
+++ b/typechecker/checker.go
@@ -0,0 +1,60 @@
+package typechecker
+
+import (
+	"errors"
+	"fmt"
+
+	"robaertschi.xyz/robaertschi/tt/ast"
+	"robaertschi.xyz/robaertschi/tt/tast"
+	"robaertschi.xyz/robaertschi/tt/token"
+)
+
+type Checker struct{}
+
+func New() *Checker {
+	return &Checker{}
+}
+
+func (c *Checker) error(t token.Token, format string, args ...any) error {
+	return fmt.Errorf("%s:%d:%d %s", t.Loc.File, t.Loc.Line, t.Loc.Col, fmt.Sprintf(format, args...))
+}
+
+func (c *Checker) CheckProgram(program ast.Program) (tast.Program, error) {
+	decls := []tast.Declaration{}
+	errs := []error{}
+
+	for _, decl := range program.Declarations {
+		decl, err := c.checkDeclaration(decl)
+		if err == nil {
+			decls = append(decls, decl)
+		} else {
+			errs = append(errs, err)
+		}
+	}
+
+	return tast.Program{Declarations: decls}, errors.Join(errs...)
+}
+
+func (c *Checker) checkDeclaration(decl ast.Declaration) (tast.Declaration, error) {
+	switch decl := decl.(type) {
+	case *ast.FunctionDeclaration:
+		body, err := c.checkExpression(decl.Body)
+
+		if err != nil {
+			return nil, err
+		}
+
+		return &tast.FunctionDeclaration{Token: decl.Token, Body: body, ReturnType: body.Type(), Name: decl.Name}, nil
+	}
+	return nil, errors.New("unhandled declaration in type checker")
+}
+
+func (c *Checker) checkExpression(expr ast.Expression) (tast.Expression, error) {
+	switch expr := expr.(type) {
+	case *ast.IntegerExpression:
+		return &tast.IntegerExpression{Token: expr.Token, Value: expr.Value}, nil
+	case *ast.ErrorExpression:
+		return nil, c.error(expr.InvalidToken, "invalid expression")
+	}
+	return nil, fmt.Errorf("unhandled expression in type checker")
+}
diff --git a/types/types.go b/types/types.go
new file mode 100644
index 0000000..995a1a2
--- /dev/null
+++ b/types/types.go
@@ -0,0 +1,36 @@
+package types
+
+type Type interface {
+	// Checks if the two types are the same
+	IsSameType(Type) bool
+	Name() string
+}
+
+type TypeId struct {
+	id   int64
+	name string
+}
+
+const (
+	I64Id int64 = iota
+)
+
+var (
+	I64 = New(I64Id, "i64")
+)
+
+func (ti *TypeId) IsSameType(t Type) bool {
+	if ti2, ok := t.(*TypeId); ok {
+		return ti.id == ti2.id
+	}
+
+	return false
+}
+
+func (ti *TypeId) Name() string {
+	return ti.name
+}
+
+func New(id int64, name string) Type {
+	return &TypeId{id: id, name: name}
+}