diff --git a/examples/collatz/collatz.q b/examples/collatz/collatz.q
index 32507a4..f7d9037 100644
--- a/examples/collatz/collatz.q
+++ b/examples/collatz/collatz.q
@@ -5,7 +5,7 @@ main() {
 	collatz(12)
 }
 
-collatz(x) {
+collatz(x Int) {
 	loop {
 		if x & 1 == 0 {
 			x /= 2
diff --git a/examples/factorial/factorial.q b/examples/factorial/factorial.q
index dab665c..885fc2f 100644
--- a/examples/factorial/factorial.q
+++ b/examples/factorial/factorial.q
@@ -4,7 +4,7 @@ main() {
 	log.number(factorial(5))
 }
 
-factorial(x) {
+factorial(x Int) -> Int {
 	if x <= 1 {
 		return 1
 	}
diff --git a/examples/fibonacci/fibonacci.q b/examples/fibonacci/fibonacci.q
index acc3e78..a071002 100644
--- a/examples/fibonacci/fibonacci.q
+++ b/examples/fibonacci/fibonacci.q
@@ -4,7 +4,7 @@ main() {
 	log.number(fibonacci(10))
 }
 
-fibonacci(x) {
+fibonacci(x Int) -> Int {
 	if x <= 1 {
 		return x
 	}
diff --git a/examples/fizzbuzz/fizzbuzz.q b/examples/fizzbuzz/fizzbuzz.q
index 9d88215..5cca2d5 100644
--- a/examples/fizzbuzz/fizzbuzz.q
+++ b/examples/fizzbuzz/fizzbuzz.q
@@ -5,7 +5,7 @@ main() {
 	fizzbuzz(15)
 }
 
-fizzbuzz(n) {
+fizzbuzz(n Int) {
 	x := 1
 
 	loop {
@@ -26,6 +26,6 @@ fizzbuzz(n) {
 	}
 }
 
-print(address, length) {
+print(address Pointer, length Int) {
 	sys.write(1, address, length)
 }
\ No newline at end of file
diff --git a/examples/gcd/gcd.q b/examples/gcd/gcd.q
index 03dead2..d952825 100644
--- a/examples/gcd/gcd.q
+++ b/examples/gcd/gcd.q
@@ -4,7 +4,7 @@ main() {
 	log.number(gcd(1071, 462))
 }
 
-gcd(a, b) {
+gcd(a Int, b Int) -> Int {
 	loop {
 		switch {
 			a == b { return a }
diff --git a/examples/hello/hello.q b/examples/hello/hello.q
index 2371295..1d9220d 100644
--- a/examples/hello/hello.q
+++ b/examples/hello/hello.q
@@ -4,6 +4,6 @@ main() {
 	print("Hello\n", 6)
 }
 
-print(address, length) {
+print(address Pointer, length Int) {
 	sys.write(1, address, length)
 }
\ No newline at end of file
diff --git a/examples/prime/prime.q b/examples/prime/prime.q
index a117bdc..836c831 100644
--- a/examples/prime/prime.q
+++ b/examples/prime/prime.q
@@ -22,7 +22,7 @@ main() {
 	}
 }
 
-isPrime(x) {
+isPrime(x Int) -> Int {
 	if x == 2 {
 		return 1
 	}
diff --git a/lib/log/number.q b/lib/log/number.q
index 86be427..93b7388 100644
--- a/lib/log/number.q
+++ b/lib/log/number.q
@@ -1,7 +1,7 @@
 import mem
 import sys
 
-number(x) {
+number(x Int) {
 	length := 20
 	buffer := mem.alloc(length)
 	address, count := itoa(x, buffer, length)
@@ -9,7 +9,7 @@ number(x) {
 	mem.free(buffer, length)
 }
 
-itoa(x, buffer, length) {
+itoa(x Int, buffer Pointer, length Int) -> (Pointer, Int) {
 	end := buffer + length
 	tmp := end
 	digit := 0
diff --git a/lib/mem/alloc.q b/lib/mem/alloc.q
index 3a6347c..747290a 100644
--- a/lib/mem/alloc.q
+++ b/lib/mem/alloc.q
@@ -1,9 +1,9 @@
 import sys
 
-alloc(length) {
+alloc(length Int) -> Pointer {
 	return sys.mmap(0, length, 0x1|0x2, 0x02|0x20|0x100)
 }
 
-free(address, length) {
+free(address Pointer, length Int) -> Int {
 	return sys.munmap(address, length)
 }
\ No newline at end of file
diff --git a/lib/sys/linux.q b/lib/sys/linux.q
index d582d1c..e6d84c2 100644
--- a/lib/sys/linux.q
+++ b/lib/sys/linux.q
@@ -1,71 +1,71 @@
-read(fd, address, length) {
+read(fd Int, address Pointer, length Int) -> Int {
 	return syscall(0, fd, address, length)
 }
 
-write(fd, address, length) {
+write(fd Int, address Pointer, length Int) -> Int {
 	return syscall(1, fd, address, length)
 }
 
-open(file, flags, mode) {
+open(file Pointer, flags Int, mode Int) -> Int {
 	return syscall(2, file, flags, mode)
 }
 
-close(fd) {
+close(fd Int) -> Int {
 	return syscall(3, fd)
 }
 
-mmap(address, length, protection, flags) {
+mmap(address Int, length Int, protection Int, flags Int) -> Int {
 	return syscall(9, address, length, protection, flags)
 }
 
-munmap(address, length) {
+munmap(address Pointer, length Int) -> Int {
 	return syscall(11, address, length)
 }
 
-clone(flags, stack) {
+clone(flags Int, stack Pointer) -> ThreadID {
 	return syscall(56, flags, stack)
 }
 
-exit(code) {
-	syscall(60, code)
+exit(status Int) {
+	syscall(60, status)
 }
 
-socket(family, type, protocol) {
+socket(family Int, type Int, protocol Int) -> Int {
 	return syscall(41, family, type, protocol)
 }
 
-accept(fd, address, length) {
+accept(fd Int, address Pointer, length Int) -> Int {
 	return syscall(43, fd, address, length)
 }
 
-bind(fd, address, length) {
+bind(fd Int, address Pointer, length Int) -> Int {
 	return syscall(49, fd, address, length)
 }
 
-listen(fd, backlog) {
+listen(fd Int, backlog Int) -> Int {
 	return syscall(50, fd, backlog)
 }
 
-getcwd(buffer, length) {
+getcwd(buffer Pointer, length Int) -> Int {
 	return syscall(79, buffer, length)
 }
 
-chdir(path) {
+chdir(path Pointer) -> Int {
 	return syscall(80, path)
 }
 
-rename(old, new) {
+rename(old Pointer, new Pointer) -> Int {
 	return syscall(82, old, new)
 }
 
-mkdir(path, mode) {
+mkdir(path Pointer, mode Int) -> Int {
 	return syscall(83, path, mode)
 }
 
-rmdir(path) {
+rmdir(path Pointer) -> Int {
 	return syscall(84, path)
 }
 
-unlink(file) {
+unlink(file Pointer) -> Int {
 	return syscall(87, file)
 }
diff --git a/src/build/errors/Base.go b/src/build/errors/Base.go
index a019d58..136c9bf 100644
--- a/src/build/errors/Base.go
+++ b/src/build/errors/Base.go
@@ -1,5 +1,26 @@
 package errors
 
+var (
+	EmptySwitch                = &Base{"Empty switch"}
+	ExpectedFunctionName       = &Base{"Expected function name"}
+	ExpectedFunctionParameters = &Base{"Expected function parameters"}
+	ExpectedFunctionDefinition = &Base{"Expected function definition"}
+	ExpectedIfBeforeElse       = &Base{"Expected an 'if' block before 'else'"}
+	InvalidNumber              = &Base{"Invalid number"}
+	InvalidExpression          = &Base{"Invalid expression"}
+	InvalidRune                = &Base{"Invalid rune"}
+	InvalidStatement           = &Base{"Invalid statement"}
+	MissingBlockStart          = &Base{"Missing '{'"}
+	MissingBlockEnd            = &Base{"Missing '}'"}
+	MissingExpression          = &Base{"Missing expression"}
+	MissingGroupStart          = &Base{"Missing '('"}
+	MissingGroupEnd            = &Base{"Missing ')'"}
+	MissingMainFunction        = &Base{"Missing main function"}
+	MissingOperand             = &Base{"Missing operand"}
+	MissingType                = &Base{"Missing type"}
+	NotImplemented             = &Base{"Not implemented"}
+)
+
 // Base is the base class for errors that have no parameters.
 type Base struct {
 	Message string
diff --git a/src/build/errors/CompileErrors.go b/src/build/errors/CompileErrors.go
deleted file mode 100644
index c0e14cb..0000000
--- a/src/build/errors/CompileErrors.go
+++ /dev/null
@@ -1,14 +0,0 @@
-package errors
-
-var (
-	EmptySwitch          = &Base{"Empty switch"}
-	ExpectedIfBeforeElse = &Base{"Expected an 'if' block before 'else'"}
-	InvalidNumber        = &Base{"Invalid number"}
-	InvalidExpression    = &Base{"Invalid expression"}
-	InvalidRune          = &Base{"Invalid rune"}
-	InvalidStatement     = &Base{"Invalid statement"}
-	MissingExpression    = &Base{"Missing expression"}
-	MissingMainFunction  = &Base{"Missing main function"}
-	MissingOperand       = &Base{"Missing operand"}
-	NotImplemented       = &Base{"Not implemented"}
-)
diff --git a/src/build/errors/ScanErrors.go b/src/build/errors/ScanErrors.go
deleted file mode 100644
index 7fe07f2..0000000
--- a/src/build/errors/ScanErrors.go
+++ /dev/null
@@ -1,11 +0,0 @@
-package errors
-
-var (
-	MissingBlockStart          = &Base{"Missing '{'"}
-	MissingBlockEnd            = &Base{"Missing '}'"}
-	MissingGroupStart          = &Base{"Missing '('"}
-	MissingGroupEnd            = &Base{"Missing ')'"}
-	ExpectedFunctionName       = &Base{"Expected function name"}
-	ExpectedFunctionParameters = &Base{"Expected function parameters"}
-	ExpectedFunctionDefinition = &Base{"Expected function definition"}
-)
diff --git a/src/build/scanner/scanFile.go b/src/build/scanner/scanFile.go
index 0f21732..59260bd 100644
--- a/src/build/scanner/scanFile.go
+++ b/src/build/scanner/scanFile.go
@@ -1,6 +1,7 @@
 package scanner
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 
@@ -153,8 +154,20 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 			return errors.New(errors.ExpectedFunctionParameters, file, tokens[i].Position)
 		}
 
+		// Return type
+		if i < len(tokens) && tokens[i].Kind == token.ReturnType {
+			for i < len(tokens) && tokens[i].Kind != token.BlockStart {
+				i++
+			}
+		}
+
 		// Function definition
 		for i < len(tokens) {
+			if tokens[i].Kind == token.ReturnType {
+				i++
+				continue
+			}
+
 			if tokens[i].Kind == token.BlockStart {
 				blockLevel++
 				i++
@@ -207,17 +220,18 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 
 		name := tokens[nameStart].Text(contents)
 		body := tokens[bodyStart:i]
-
 		function := core.NewFunction(pkg, name, file, body)
 		parameters := tokens[paramsStart:paramsEnd]
 		count := 0
 
 		err := expression.EachParameter(parameters, func(tokens token.List) error {
-			if len(tokens) != 1 {
-				return errors.New(errors.NotImplemented, file, tokens[0].Position)
+			if len(tokens) < 2 {
+				return errors.New(errors.MissingType, file, tokens[0].End())
 			}
 
 			name := tokens[0].Text(contents)
+			dataType := tokens[1].Text(contents)
+			fmt.Println(dataType)
 			register := x64.CallRegisters[count]
 			uses := token.Count(function.Body, contents, token.Identifier, name)
 
diff --git a/src/build/token/Kind.go b/src/build/token/Kind.go
index fa97224..e0c23f8 100644
--- a/src/build/token/Kind.go
+++ b/src/build/token/Kind.go
@@ -18,6 +18,7 @@ const (
 	BlockEnd                    // }
 	ArrayStart                  // [
 	ArrayEnd                    // ]
+	ReturnType                  // ->
 	_operators                  // <operators>
 	Add                         // +
 	Sub                         // -
diff --git a/src/build/token/Tokenize.go b/src/build/token/Tokenize.go
index 85d3eba..2e15e25 100644
--- a/src/build/token/Tokenize.go
+++ b/src/build/token/Tokenize.go
@@ -30,9 +30,17 @@ func Tokenize(buffer []byte) List {
 			if len(tokens) == 0 || tokens[len(tokens)-1].IsOperator() || tokens[len(tokens)-1].IsExpressionStart() || tokens[len(tokens)-1].IsKeyword() {
 				tokens = append(tokens, Token{Kind: Negate, Position: i, Length: 1})
 			} else {
-				if i+1 < Position(len(buffer)) && buffer[i+1] == '=' {
-					tokens = append(tokens, Token{Kind: SubAssign, Position: i, Length: 2})
-					i++
+				if i+1 < Position(len(buffer)) {
+					switch buffer[i+1] {
+					case '=':
+						tokens = append(tokens, Token{Kind: SubAssign, Position: i, Length: 2})
+						i++
+					case '>':
+						tokens = append(tokens, Token{Kind: ReturnType, Position: i, Length: 2})
+						i++
+					default:
+						tokens = append(tokens, Token{Kind: Sub, Position: i, Length: 1})
+					}
 				} else {
 					tokens = append(tokens, Token{Kind: Sub, Position: i, Length: 1})
 				}
@@ -212,10 +220,12 @@ func Tokenize(buffer []byte) List {
 					kind = Add
 				case "+=":
 					kind = AddAssign
-				case "-":
-					kind = Sub
-				case "-=":
-					kind = SubAssign
+				// case "-":
+				// 	kind = Sub
+				// case "-=":
+				// 	kind = SubAssign
+				// case "->":
+				// 	kind = ReturnType
 				case ".":
 					kind = Period
 				case "/":
diff --git a/tests/errors/UnknownIdentifier3.q b/tests/errors/UnknownIdentifier3.q
index a7387e2..4cc4927 100644
--- a/tests/errors/UnknownIdentifier3.q
+++ b/tests/errors/UnknownIdentifier3.q
@@ -2,6 +2,6 @@ main() {
 	x := 1 + f(x)
 }
 
-f(x) {
+f(x Int) -> Int {
 	return x
 }
\ No newline at end of file
diff --git a/tests/programs/branch.q b/tests/programs/branch.q
index 9e6f514..171002b 100644
--- a/tests/programs/branch.q
+++ b/tests/programs/branch.q
@@ -70,14 +70,14 @@ main() {
 	if x == 0 {
 		sys.exit(0)
 	}
-	
+
 	sys.exit(1)
 }
 
-inc(x) {
+inc(x Int) -> Int {
 	return x + 1
 }
 
-dec(x) {
+dec(x Int) -> Int {
 	return x - 1
 }
\ No newline at end of file
diff --git a/tests/programs/chained-calls.q b/tests/programs/chained-calls.q
index e62a02e..cd3c519 100644
--- a/tests/programs/chained-calls.q
+++ b/tests/programs/chained-calls.q
@@ -2,6 +2,6 @@ main() {
 	assert f(1) + f(2) + f(3) == 9
 }
 
-f(x) {
+f(x Int) -> Int {
 	return x + 1
 }
\ No newline at end of file
diff --git a/tests/programs/64-bit.q b/tests/programs/int64.q
similarity index 100%
rename from tests/programs/64-bit.q
rename to tests/programs/int64.q
diff --git a/tests/programs/loop-lifetime.q b/tests/programs/loop-lifetime.q
index 1262b7d..d30489e 100644
--- a/tests/programs/loop-lifetime.q
+++ b/tests/programs/loop-lifetime.q
@@ -12,6 +12,6 @@ main() {
 	}
 }
 
-f(x) {
+f(x Int) -> Int {
 	return x
 }
\ No newline at end of file
diff --git a/tests/programs/math.q b/tests/programs/math.q
index 6fd1712..2de1b1b 100644
--- a/tests/programs/math.q
+++ b/tests/programs/math.q
@@ -4,10 +4,10 @@ main() {
 	assert result == 10
 }
 
-div(x, y) {
+div(x Int, y Int) -> Int {
 	return x / y
 }
 
-div10(x) {
+div10(x Int) -> Int {
 	return x / 10
 }
\ No newline at end of file
diff --git a/tests/programs/negation.q b/tests/programs/negation.q
index 709f995..089d4d7 100644
--- a/tests/programs/negation.q
+++ b/tests/programs/negation.q
@@ -5,6 +5,6 @@ main() {
 	assert neg(256) == -256
 }
 
-neg(x) {
+neg(x Int) -> Int {
 	return -x
 }
\ No newline at end of file
diff --git a/tests/programs/nested-calls.q b/tests/programs/nested-calls.q
index a23e15c..e555b99 100644
--- a/tests/programs/nested-calls.q
+++ b/tests/programs/nested-calls.q
@@ -2,6 +2,6 @@ main() {
 	assert f(f(f(1))) == 4
 }
 
-f(x) {
+f(x Int) -> Int {
 	return x + 1
 }
\ No newline at end of file
diff --git a/tests/programs/op-assign.q b/tests/programs/op-assign.q
index 496b3a5..257b6ac 100644
--- a/tests/programs/op-assign.q
+++ b/tests/programs/op-assign.q
@@ -2,7 +2,7 @@ main() {
 	f(10)
 }
 
-f(new) {
+f(new Int) {
 	old := new
 	new -= 1
 	assert new != old
diff --git a/tests/programs/param-multi.q b/tests/programs/param-multi.q
index 1267e94..0e19661 100644
--- a/tests/programs/param-multi.q
+++ b/tests/programs/param-multi.q
@@ -2,11 +2,11 @@ main() {
 	assert f(1, 2, 3) == 21
 }
 
-f(x, y, z) {
+f(x Int, y Int, z Int) -> Int {
 	w := g(4, 5, 6)
 	return x + y + z + w
 }
 
-g(x, y, z) {
+g(x Int, y Int, z Int) -> Int {
 	return x + y + z
 }
\ No newline at end of file
diff --git a/tests/programs/param-order.q b/tests/programs/param-order.q
index a36d6e8..2b8ef0f 100644
--- a/tests/programs/param-order.q
+++ b/tests/programs/param-order.q
@@ -2,11 +2,11 @@ main() {
 	f(1, 2, 3, 4, 5, 6)
 }
 
-f(a, b, c, d, e, f) {
-	return g(f, e, d, c, b, a)
+f(a Int, b Int, c Int, d Int, e Int, f Int) {
+	g(f, e, d, c, b, a)
 }
 
-g(a, b, c, d, e, f) {
+g(a Int, b Int, c Int, d Int, e Int, f Int) {
 	assert a == 6
 	assert b == 5
 	assert c == 4
diff --git a/tests/programs/param.q b/tests/programs/param.q
index 11ee501..5e1628d 100644
--- a/tests/programs/param.q
+++ b/tests/programs/param.q
@@ -2,11 +2,11 @@ main() {
 	assert f(1) == 3
 }
 
-f(x) {
+f(x Int) -> Int {
 	y := g()
 	return x + y
 }
 
-g() {
+g() -> Int {
 	return 2
 }
\ No newline at end of file
diff --git a/tests/programs/return-multi.q b/tests/programs/return-multi.q
index 47e72be..be81c42 100644
--- a/tests/programs/return-multi.q
+++ b/tests/programs/return-multi.q
@@ -15,14 +15,14 @@ main() {
 	assert i == 1 + 4
 }
 
-reverse2(a, b) {
+reverse2(a Int, b Int) -> (Int, Int) {
 	return b, a
 }
 
-reverse3(a, b, c) {
+reverse3(a Int, b Int, c Int) -> (Int, Int, Int) {
 	return c, b, a
 }
 
-mix4(a, b, c, d) {
+mix4(a Int, b Int, c Int, d Int) -> (Int, Int, Int, Int) {
 	return d + a, c + b, b + c, a + d
 }
\ No newline at end of file
diff --git a/tests/programs/return.q b/tests/programs/return.q
index f5e13d5..36095bc 100644
--- a/tests/programs/return.q
+++ b/tests/programs/return.q
@@ -2,10 +2,10 @@ main() {
 	assert f(2) == 6
 }
 
-f(x) {
+f(x Int) -> Int {
 	return x + 1 + g(x)
 }
 
-g(x) {
+g(x Int) -> Int {
 	return x + 1
 }
\ No newline at end of file
diff --git a/tests/programs/reuse.q b/tests/programs/reuse.q
index 010f747..4658edb 100644
--- a/tests/programs/reuse.q
+++ b/tests/programs/reuse.q
@@ -2,6 +2,6 @@ main() {
 	assert f(1) == 3
 }
 
-f(x) {
+f(x Int) -> Int {
 	return x + 1 + x
 }
\ No newline at end of file
diff --git a/tests/programs/square-sum.q b/tests/programs/square-sum.q
index 04b1237..a9c0850 100644
--- a/tests/programs/square-sum.q
+++ b/tests/programs/square-sum.q
@@ -2,6 +2,6 @@ main() {
 	assert f(2, 3) == 25
 }
 
-f(x, y) {
+f(x Int, y Int) -> Int {
 	return (x + y) * (x + y)
 }
\ No newline at end of file
diff --git a/tests/programs_test.go b/tests/programs_test.go
index 39c5677..7e002a7 100644
--- a/tests/programs_test.go
+++ b/tests/programs_test.go
@@ -40,7 +40,7 @@ var programs = []struct {
 	{"modulo", "", "", 0},
 	{"modulo-assign", "", "", 0},
 	{"div-split", "", "", 0},
-	{"64-bit", "", "", 0},
+	{"int64", "", "", 0},
 	{"negative", "", "", 0},
 	{"negation", "", "", 0},
 	{"square-sum", "", "", 0},