diff --git a/ast/ast.go b/ast/ast.go
index b81826c..2845fc8 100644
--- a/ast/ast.go
+++ b/ast/ast.go
@@ -64,9 +64,10 @@ type FunctionDeclaration struct {
 	Body       Expression
 	Name       string
 	Parameters []Parameter
+	ReturnType Type
 }
 
-func ArgsToString(args []Parameter) string {
+func ParamsToString(args []Parameter) string {
 	var b strings.Builder
 
 	for _, arg := range args {
@@ -80,7 +81,7 @@ func (fd *FunctionDeclaration) declarationNode()     {}
 func (fd *FunctionDeclaration) TokenLiteral() string { return fd.Token.Literal }
 func (fd *FunctionDeclaration) Tok() token.Token     { return fd.Token }
 func (fd *FunctionDeclaration) String() string {
-	return fmt.Sprintf("fn %v(%v) = %v;", fd.Name, ArgsToString(fd.Parameters), fd.Body.String())
+	return fmt.Sprintf("fn %v(%v): %v = %v;", fd.Name, ParamsToString(fd.Parameters), fd.ReturnType, fd.Body.String())
 }
 
 // Represents a Expression that we failed to parse
diff --git a/parser/parser.go b/parser/parser.go
index 30b03d9..320a874 100644
--- a/parser/parser.go
+++ b/parser/parser.go
@@ -236,6 +236,14 @@ func (p *Parser) parseDeclaration() ast.Declaration {
 	if ok, _ := p.expectPeek(token.CloseParen); !ok {
 		return nil
 	}
+	if ok, _ := p.expectPeek(token.Colon); !ok {
+		return nil
+	}
+	p.nextToken()
+	t, ok := p.parseType()
+	if !ok {
+		return nil
+	}
 	if ok, _ := p.expectPeek(token.Equal); !ok {
 		return nil
 	}
@@ -251,6 +259,7 @@ func (p *Parser) parseDeclaration() ast.Declaration {
 		Name:       name,
 		Body:       expr,
 		Parameters: params,
+		ReturnType: t,
 	}
 }
 
diff --git a/tast/tast.go b/tast/tast.go
index 1902f13..3d9ff6e 100644
--- a/tast/tast.go
+++ b/tast/tast.go
@@ -52,12 +52,12 @@ func (p *Program) String() string {
 	return builder.String()
 }
 
-type Argument struct {
+type Parameter struct {
 	Name string
 	Type types.Type
 }
 
-func ArgsToString(args []Argument) string {
+func ArgsToString(args []Parameter) string {
 	var b strings.Builder
 
 	for _, arg := range args {
@@ -71,7 +71,7 @@ type FunctionDeclaration struct {
 	Token      token.Token // The token.FN
 	Body       Expression
 	Name       string
-	Args       []Argument
+	Parameters []Parameter
 	ReturnType types.Type
 }
 
@@ -81,7 +81,7 @@ func (fd *FunctionDeclaration) declarationNode()     {}
 func (fd *FunctionDeclaration) TokenLiteral() string { return fd.Token.Literal }
 func (fd *FunctionDeclaration) Tok() token.Token     { return fd.Token }
 func (fd *FunctionDeclaration) String() string {
-	return fmt.Sprintf("fn %v(%v): %v = %v;", fd.Name, ArgsToString(fd.Args), fd.ReturnType.Name(), fd.Body.String())
+	return fmt.Sprintf("fn %v(%v): %v = %v;", fd.Name, ArgsToString(fd.Parameters), fd.ReturnType.Name(), fd.Body.String())
 }
 
 type IntegerExpression struct {
diff --git a/test.tt b/test.tt
index b0bbf95..682d676 100644
--- a/test.tt
+++ b/test.tt
@@ -1,4 +1,4 @@
-fn main() = {
+fn main(): i64 = {
     i := 5;
 
 
@@ -11,7 +11,7 @@ fn main() = {
     test2(3)
 };
 
-fn test2(hello: i64) = {
+fn test2(hello: i64): i64 = {
     hello // Comment test
 };
 
diff --git a/ttir/emit.go b/ttir/emit.go
index f0b749f..aab4be3 100644
--- a/ttir/emit.go
+++ b/ttir/emit.go
@@ -47,7 +47,7 @@ func emitFunction(function *tast.FunctionDeclaration) *Function {
 
 	arguments := []string{}
 
-	for _, arg := range function.Args {
+	for _, arg := range function.Parameters {
 		arguments = append(arguments, arg.Name)
 	}
 
diff --git a/typechecker/check.go b/typechecker/check.go
index 35f73d0..b660a7c 100644
--- a/typechecker/check.go
+++ b/typechecker/check.go
@@ -12,6 +12,16 @@ import (
 
 type Variables map[string]types.Type
 
+func copyVars(vars Variables) Variables {
+	newVars := make(Variables)
+
+	for k, v := range vars {
+		newVars[k] = v
+	}
+
+	return newVars
+}
+
 type Checker struct {
 	foundMain         bool
 	functionVariables map[string]Variables
@@ -152,6 +162,22 @@ func (c *Checker) checkExpression(vars Variables, expr tast.Expression) error {
 		return nil
 	case *tast.VariableReference:
 		return nil
+	case *tast.FunctionCall:
+		functionType := vars[expr.Identifier].(*types.FunctionType)
+		if len(expr.Arguments) != len(functionType.Parameters) {
+			return c.error(expr.Token, "invalid amount of arguments for function %q, expected %d but got %d", expr.Identifier, len(functionType.Parameters), len(expr.Arguments))
+		}
+
+		errs := []error{}
+
+		for i, param := range functionType.Parameters {
+			e := expr.Arguments[i]
+			if !e.Type().IsSameType(param) {
+				errs = append(errs, c.error(e.Tok(), "invalid type for parameter, expected %q but got %q", param.Name(), e.Type().Name()))
+			}
+		}
+
+		return errors.Join(errs...)
 	default:
 		panic(fmt.Sprintf("unexpected tast.Expression: %#v", expr))
 	}
diff --git a/typechecker/infer.go b/typechecker/infer.go
index 0f2825f..328672f 100644
--- a/typechecker/infer.go
+++ b/typechecker/infer.go
@@ -13,9 +13,41 @@ func (c *Checker) inferTypes(program *ast.Program) (*tast.Program, error) {
 	c.functionVariables = make(map[string]Variables)
 	decls := []tast.Declaration{}
 	errs := []error{}
+	vars := make(Variables)
+
+	funcToParams := make(map[string][]tast.Parameter)
 
 	for _, decl := range program.Declarations {
-		decl, err := c.inferDeclaration(decl)
+		switch decl := decl.(type) {
+		case *ast.FunctionDeclaration:
+			parameters := []tast.Parameter{}
+			for _, param := range decl.Parameters {
+				t, ok := types.From(param.Type)
+				if !ok {
+					return nil, c.error(decl.Token, "could not find the type %q for argument %q", param.Type, param.Name)
+				}
+				vars[param.Name] = t
+				parameters = append(parameters, tast.Parameter{Name: param.Name, Type: t})
+			}
+
+			t, ok := types.From(decl.ReturnType)
+			if !ok {
+				return nil, c.error(decl.Token, "invalid type %q", decl.ReturnType)
+			}
+
+			parameterTypes := []types.Type{}
+
+			for _, param := range parameters {
+				parameterTypes = append(parameterTypes, param.Type)
+			}
+
+			vars[decl.Name] = &types.FunctionType{ReturnType: t, Parameters: parameterTypes}
+			funcToParams[decl.Name] = parameters
+		}
+	}
+
+	for _, decl := range program.Declarations {
+		decl, err := c.inferDeclaration(funcToParams, copyVars(vars), decl)
 		if err == nil {
 			decls = append(decls, decl)
 		} else {
@@ -26,20 +58,9 @@ func (c *Checker) inferTypes(program *ast.Program) (*tast.Program, error) {
 	return &tast.Program{Declarations: decls}, errors.Join(errs...)
 }
 
-func (c *Checker) inferDeclaration(decl ast.Declaration) (tast.Declaration, error) {
+func (c *Checker) inferDeclaration(funcToParams map[string][]tast.Parameter, vars Variables, decl ast.Declaration) (tast.Declaration, error) {
 	switch decl := decl.(type) {
 	case *ast.FunctionDeclaration:
-		vars := make(Variables)
-		parameters := []tast.Parameter{}
-		for _, param := range decl.Parameters {
-			t, ok := types.From(param.Type)
-			if !ok {
-				return nil, c.error(decl.Token, "could not find the type %q for argument %q", param.Type, param.Name)
-			}
-			vars[param.Name] = t
-			parameters = append(parameters, tast.Parameter{Name: param.Name, Type: t})
-		}
-		// vars[decl.Name] = &types.FunctionType{ReturnType: }
 		body, err := c.inferExpression(vars, decl.Body)
 		c.functionVariables[decl.Name] = vars
 
@@ -47,7 +68,7 @@ func (c *Checker) inferDeclaration(decl ast.Declaration) (tast.Declaration, erro
 			return nil, err
 		}
 
-		return &tast.FunctionDeclaration{Token: decl.Token, Args: parameters, Body: body, ReturnType: body.Type(), Name: decl.Name}, nil
+		return &tast.FunctionDeclaration{Token: decl.Token, Parameters: funcToParams[decl.Name], Body: body, ReturnType: vars[decl.Name], Name: decl.Name}, nil
 	}
 	return nil, errors.New("unhandled declaration in type inferer")
 }
@@ -181,6 +202,30 @@ func (c *Checker) inferExpression(vars Variables, expr ast.Expression) (tast.Exp
 		if !ok {
 			return fc, c.error(expr.Token, "could not get type for function %q", fc.Identifier)
 		}
+
+		funcType, ok := t.(*types.FunctionType)
+		if !ok {
+			return fc, c.error(expr.Token, "tried to call non function variable %q with type %q", expr.Identifier, t.Name())
+		}
+
+		fc.ReturnType = funcType.ReturnType
+
+		args := []tast.Expression{}
+		errs := []error{}
+
+		for _, arg := range expr.Arguments {
+			inferredArg, err := c.inferExpression(vars, arg)
+			errs = append(errs, err)
+
+			if err == nil {
+				args = append(args, inferredArg)
+			}
+		}
+
+		fc.Arguments = args
+
+		return fc, errors.Join(errs...)
+
 	default:
 		panic(fmt.Sprintf("unexpected ast.Expression: %#v", expr))
 	}
diff --git a/typechecker/variable_resolution.go b/typechecker/variable_resolution.go
index 1730308..ccdc673 100644
--- a/typechecker/variable_resolution.go
+++ b/typechecker/variable_resolution.go
@@ -69,6 +69,15 @@ func (s *Scope) SetUniq(name string) string {
 
 func VarResolve(p *ast.Program) (map[string]Scope, error) {
 	functionToScope := make(map[string]Scope)
+	functions := Scope{Variables: make(map[string]Var)}
+
+	for _, d := range p.Declarations {
+		switch d := d.(type) {
+		case *ast.FunctionDeclaration:
+			functions.Set(d.Name, d.Name)
+		default:
+		}
+	}
 
 	for _, d := range p.Declarations {
 		switch d := d.(type) {
@@ -78,8 +87,7 @@ func VarResolve(p *ast.Program) (map[string]Scope, error) {
 				return functionToScope, errorf(d.Token, "duplicate function name %q", d.Name)
 			}
 
-			s := Scope{Variables: make(map[string]Var)}
-			s.Set(d.Name, d.Name)
+			s := copyScope(&functions)
 			for i, param := range d.Parameters {
 				uniq := s.SetUniq(param.Name)
 				d.Parameters[i].Name = uniq