diff --git a/README.md b/README.md
index 2b6cfb4..bd50be9 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,12 @@ Build a Linux ELF executable from `examples/hello`:
 ./examples/hello/hello
 ```
 
+To produce verbose output, add the `-v` flag:
+
+```shell
+./q build examples/hello -v
+```
+
 ## Documentation
 
 ### [main.go](main.go)
diff --git a/examples/hello/hello.q b/examples/hello/hello.q
index 2981aa1..6c3365a 100644
--- a/examples/hello/hello.q
+++ b/examples/hello/hello.q
@@ -1,7 +1,7 @@
 main() {
-	let write = 1
-	let exit = 60
-	let stdout = 1
+	write := 1
+	exit := 60
+	stdout := 1
 
 	syscall(write, stdout, 4194305, 3)
 	syscall(exit, 0)
diff --git a/src/build/Function.go b/src/build/Function.go
index 2a2d670..83367de 100644
--- a/src/build/Function.go
+++ b/src/build/Function.go
@@ -18,6 +18,7 @@ type Function struct {
 	Body      token.List
 	Variables map[string]*Variable
 	Assembler asm.Assembler
+	Error     error
 }
 
 // Compile turns a function into machine code.
@@ -27,35 +28,53 @@ func (f *Function) Compile() {
 	}
 
 	for _, line := range f.Lines() {
-		if config.Verbose {
-			fmt.Println("[line]", line)
-		}
-
 		if len(line) == 0 {
 			continue
 		}
 
-		if line[0].Kind == token.Keyword {
-			switch line[0].Text() {
-			case "let":
-				name := line[1].Text()
-				value := line[3:]
-
-				if config.Verbose {
-					fmt.Println("[variable]", name, value)
-				}
-
-				f.Variables[name] = &Variable{
-					Value:   line[3:],
-					IsConst: true,
-				}
-
-			case "return":
-				f.Assembler.Return()
-			}
+		if config.Verbose {
+			fmt.Println("[line]", line)
 		}
 
-		if line[0].Kind == token.Identifier && line[0].Text() == "syscall" {
+		err := f.compileInstruction(line)
+
+		if err != nil {
+			ansi.Red.Println(err)
+			return
+		}
+	}
+
+	f.Assembler.Return()
+}
+
+// compileInstruction compiles a single instruction.
+func (f *Function) compileInstruction(line token.List) error {
+	switch line[0].Kind {
+	case token.Keyword:
+		switch line[0].Text() {
+		case "return":
+			f.Assembler.Return()
+		}
+
+	case token.Identifier:
+		if len(line) >= 2 && line[1].Kind == token.Operator && line[1].Text() == ":=" {
+			name := line[0].Text()
+			value := line[2:]
+
+			if config.Verbose {
+				fmt.Println("[variable]", name, value)
+			}
+
+			f.Variables[name] = &Variable{
+				Value:   line[2:],
+				IsConst: true,
+			}
+
+			return nil
+		}
+
+		switch line[0].Text() {
+		case "syscall":
 			paramTokens := line[2 : len(line)-1]
 			start := 0
 			i := 0
@@ -78,7 +97,11 @@ func (f *Function) Compile() {
 				switch list[0].Kind {
 				case token.Identifier:
 					name := list[0].Text()
-					variable := f.Variables[name]
+					variable, exists := f.Variables[name]
+
+					if !exists {
+						return fmt.Errorf("Unknown identifier '%s'", name)
+					}
 
 					if !variable.IsConst {
 						panic("Not implemented yet")
@@ -101,7 +124,7 @@ func (f *Function) Compile() {
 		}
 	}
 
-	f.Assembler.Return()
+	return nil
 }
 
 // Lines returns the lines in the function body.
diff --git a/src/build/token/Keywords.go b/src/build/token/Keywords.go
index a8eec3c..adb8de7 100644
--- a/src/build/token/Keywords.go
+++ b/src/build/token/Keywords.go
@@ -2,6 +2,5 @@ package token
 
 // Keywords defines the keywords used in the language.
 var Keywords = map[string]bool{
-	"let":    true,
 	"return": true,
 }
diff --git a/src/build/token/List.go b/src/build/token/List.go
index 9cef514..be3a1ff 100644
--- a/src/build/token/List.go
+++ b/src/build/token/List.go
@@ -13,7 +13,7 @@ func (list List) String() string {
 	var last Token
 
 	for _, t := range list {
-		if last.Kind == Keyword || last.Kind == Separator {
+		if last.Kind == Keyword || last.Kind == Separator || last.Kind == Operator || t.Kind == Operator {
 			builder.WriteByte(' ')
 		}
 
diff --git a/src/build/token/Tokenize.go b/src/build/token/Tokenize.go
index 53cdbb6..ac5d8b0 100644
--- a/src/build/token/Tokenize.go
+++ b/src/build/token/Tokenize.go
@@ -70,9 +70,6 @@ func Tokenize(buffer []byte) List {
 		case ']':
 			tokens = append(tokens, Token{ArrayEnd, i, arrayEndBytes})
 
-		case '=', ':', '+', '-', '*', '/', '<', '>', '!':
-			tokens = append(tokens, Token{Operator, i, buffer[i : i+1]})
-
 		// Separator
 		case ',':
 			tokens = append(tokens, Token{Separator, i, separatorBytes})
@@ -122,6 +119,24 @@ func Tokenize(buffer []byte) List {
 
 				continue
 			}
+
+			// Operators
+			if isOperator(buffer[i]) {
+				position := i
+				i++
+
+				for i < len(buffer) && isOperator(buffer[i]) {
+					i++
+				}
+
+				tokens = append(tokens, Token{
+					Operator,
+					position,
+					buffer[position:i],
+				})
+
+				continue
+			}
 		}
 
 		i++