diff --git a/lexer/lexer.go b/lexer/lexer.go
index 03565b6..b723f75 100644
--- a/lexer/lexer.go
+++ b/lexer/lexer.go
@@ -29,6 +29,8 @@ func (l *Lexer) readChar() {
 func (l *Lexer) NextToken() token.Token {
 	var tok token.Token
 
+	l.skipWhitespace()
+
 	switch l.ch {
 	case '=':
 		tok = newToken(token.ASSIGN, l.ch)
@@ -49,12 +51,56 @@ func (l *Lexer) NextToken() token.Token {
 	case 0:
 		tok.Literal = ""
 		tok.Type = token.EOF
+	default:
+		if isLetter(l.ch) {
+			tok.Literal = l.readIdentifier()
+			tok.Type = token.LookupIdent(tok.Literal)
+			return tok
+		} else if isDigit(l.ch) {
+			tok.Type = token.INT
+			tok.Literal = l.readNumber()
+			return tok
+		} else {
+			tok = newToken(token.ILLEGAL, l.ch)
+		}
 	}
 
 	l.readChar()
 	return tok
 }
 
+func (l *Lexer) readNumber() string {
+	position := l.position
+	for isDigit(l.ch) {
+		l.readChar()
+	}
+
+	return l.input[position:l.position]
+}
+
+func (l *Lexer) readIdentifier() string {
+	position := l.position
+	for isLetter(l.ch) {
+		l.readChar()
+	}
+
+	return l.input[position:l.position]
+}
+
+func (l *Lexer) skipWhitespace() {
+	for l.ch == ' ' || l.ch == '\t' || l.ch == '\n' || l.ch == '\r' {
+		l.readChar()
+	}
+}
+
+func isLetter(ch byte) bool {
+	return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_'
+}
+
+func isDigit(ch byte) bool {
+	return '0' <= ch && ch <= '9'
+}
+
 func newToken(tokenType token.TokenType, ch byte) token.Token {
 	return token.Token{Type: tokenType, Literal: string(ch)}
 }
diff --git a/lexer/lexer_test.go b/lexer/lexer_test.go
index 5df7544..b01fba2 100644
--- a/lexer/lexer_test.go
+++ b/lexer/lexer_test.go
@@ -6,21 +6,56 @@ import (
 	"git.cybercyst.me/monkeylang/token"
 )
 
-func TextNextToken(t *testing.T) {
-	input := `=+(){},;`
+func TestNextToken(t *testing.T) {
+	input := `let five = 5;
+let ten = 10;
+let add = fn(x, y) {
+  x + y;
+};
+let result = add(five, ten);
+`
 
 	tests := []struct {
 		expectedType    token.TokenType
 		expectedLiteral string
 	}{
+		{token.LET, "let"},
+		{token.IDENT, "five"},
 		{token.ASSIGN, "="},
-		{token.PLUS, "+"},
+		{token.INT, "5"},
+		{token.SEMICOLON, ";"},
+		{token.LET, "let"},
+		{token.IDENT, "ten"},
+		{token.ASSIGN, "="},
+		{token.INT, "10"},
+		{token.SEMICOLON, ";"},
+		{token.LET, "let"},
+		{token.IDENT, "add"},
+		{token.ASSIGN, "="},
+		{token.FUNCTION, "fn"},
 		{token.LPAREN, "("},
+		{token.IDENT, "x"},
+		{token.COMMA, ","},
+		{token.IDENT, "y"},
 		{token.RPAREN, ")"},
 		{token.LBRACE, "{"},
-		{token.RBRACE, "}"},
-		{token.COMMA, ","},
+		{token.IDENT, "x"},
+		{token.PLUS, "+"},
+		{token.IDENT, "y"},
 		{token.SEMICOLON, ";"},
+		{token.RBRACE, "}"},
+		{token.SEMICOLON, ";"},
+		{token.LET, "let"},
+		{token.IDENT, "result"},
+		{token.ASSIGN, "="},
+		{token.IDENT, "add"},
+		{token.LPAREN, "("},
+		{token.IDENT, "five"},
+		{token.COMMA, ","},
+		{token.IDENT, "ten"},
+		{token.RPAREN, ")"},
+		{token.SEMICOLON, ";"},
+		{token.EOF, ""},
 	}
 
 	l := New(input)
diff --git a/token/token.go b/token/token.go
index 23c9a9a..d050230 100644
--- a/token/token.go
+++ b/token/token.go
@@ -29,3 +29,15 @@ const (
 	FUNCTION = "FUNCTION"
 	LET      = "LET"
 )
+
+var keywords = map[string]TokenType{
+	"fn":  FUNCTION,
+	"let": LET,
+}
+
+func LookupIdent(ident string) TokenType {
+	if tokType, ok := keywords[ident]; ok {
+		return tokType
+	}
+	return IDENT
+}