diff --git a/PLAN.md b/PLAN.md
index df56f19..daee6eb 100644
--- a/PLAN.md
+++ b/PLAN.md
@@ -8,10 +8,10 @@ A Rust-flavored, C-targeting language - built pipeline-first.
 ## Phase 1 - Lexer
 
 - [x] Define token enum (int literal, bool literal, ident, keywords, operators, punctuation)
-- [ ] Implement character-by-character scanner loop
-- [ ] Handle whitespace & single-line comments (`//`)
-- [ ] Produce source spans (file, line, col) on every token
-- [ ] Unit-test: known inputs → expected token streams
+- [x] Implement character-by-character scanner loop
+- [x] Handle whitespace & single-line comments (`//`)
+- [x] Produce source spans (file, line, col) on every token
+- [x] Unit-test: known inputs → expected token streams
 
 ## Phase 2 - Parser
 
diff --git a/src/frontend/lexer.rs b/src/frontend/lexer.rs
new file mode 100644
index 0000000..9f0ea69
--- /dev/null
+++ b/src/frontend/lexer.rs
@@ -0,0 +1,286 @@
+use super::token::{Span, Token, TokenKind};
+use std::{iter::Peekable, str::Chars};
+
+/// Splits a source string into a sequence of [`Token`]s to be consumed by the parser.
+///
+/// Internally tracks a [`Peekable`] char iterator and a byte-offset cursor
+/// to produce accurate source spans on every token.
+pub struct Lexer<'src> {
+    source: &'src str,
+    chars: Peekable<Chars<'src>>,
+    cursor: usize,
+}
+
+impl<'src> Lexer<'src> {
+    /// Creates a new lexer with given input source.
+    pub fn new(source: &'src str) -> Self {
+        let chars = source.chars().peekable();
+
+        Self {
+            source,
+            chars,
+            cursor: 0,
+        }
+    }
+
+    /// Advances the lexer updating the cursor position.
+    fn advance(&mut self) -> Option<char> {
+        let ch = self.chars.next()?;
+        self.cursor += ch.len_utf8();
+        Some(ch)
+    }
+
+    /// Advances the lexer as long as the predicate matches.
+    fn advance_while(&mut self, predicate: impl Fn(char) -> bool) {
+        while self.peek().is_some_and(&predicate) {
+            self.advance();
+        }
+    }
+
+    /// Peeks at the next character without advancing.
+    fn peek(&mut self) -> Option<char> {
+        self.chars.peek().copied()
+    }
+
+    /// Skip whitespace and line comments.
+    fn skip_whitespace_and_comments(&mut self) {
+        loop {
+            self.advance_while(char::is_whitespace);
+
+            if self.source[self.cursor..].starts_with("//") {
+                self.advance_while(|ch| ch != '\n');
+                continue;
+            }
+
+            break;
+        }
+    }
+
+    /// Lexes the next identifier or keyword.
+    fn lex_identifier_or_keyword(&mut self) -> TokenKind {
+        let start = self.cursor;
+
+        self.advance_while(|ch| matches!(ch, 'a'..='z' | 'A'..='Z' | '0'..='9' | '_'));
+
+        match &self.source[start..self.cursor] {
+            "fn" => TokenKind::Fn,
+            "return" => TokenKind::Return,
+
+            "i8" => TokenKind::I8,
+            "i16" => TokenKind::I16,
+            "i32" => TokenKind::I32,
+            "i64" => TokenKind::I64,
+            "u8" => TokenKind::U8,
+            "u16" => TokenKind::U16,
+            "u32" => TokenKind::U32,
+            "u64" => TokenKind::U64,
+            "bool" => TokenKind::Bool,
+
+            "true" | "false" => TokenKind::BooleanLit,
+
+            _ => TokenKind::Identifier,
+        }
+    }
+
+    /// Lexes the next number.
+    fn lex_number(&mut self) -> TokenKind {
+        let radix = match (self.advance(), self.peek()) {
+            (Some('0'), Some('x' | 'X')) => {
+                self.advance();
+                16
+            }
+            (Some('0'), Some('o' | 'O')) => {
+                self.advance();
+                8
+            }
+            (Some('0'), Some('b' | 'B')) => {
+                self.advance();
+                2
+            }
+            _ => 10,
+        };
+
+        self.advance_while(|ch| ch.is_digit(radix));
+
+        TokenKind::IntegerLit
+    }
+
+    /// Lexes the next [Token].
+    pub fn next_token(&mut self) -> Option<Token<'src>> {
+        self.skip_whitespace_and_comments();
+
+        let start = self.cursor;
+
+        macro_rules! token {
+            ($kind:expr) => {{
+                self.advance();
+                $kind
+            }};
+
+            ($default:expr, $($ch:expr => $kind:expr),+ $(,)?) => {{
+                self.advance();
+
+                match self.peek() {
+                    $(
+                        Some($ch) => {
+                            self.advance();
+                            $kind
+                        }
+                    )+
+                    _ => $default,
+                }
+            }};
+        }
+
+        let kind = match self.peek()? {
+            'a'..='z' | 'A'..='Z' | '_' => self.lex_identifier_or_keyword(),
+            '0'..='9' => self.lex_number(),
+
+            '+' => token!(TokenKind::Plus),
+            '-' => token!(TokenKind::Minus, '>' => TokenKind::Arrow),
+            '*' => token!(TokenKind::Star),
+            '/' => token!(TokenKind::Slash),
+            '%' => token!(TokenKind::Percent),
+
+            '.' => token!(TokenKind::Dot),
+            ',' => token!(TokenKind::Comma),
+            ':' => token!(TokenKind::Colon),
+            ';' => token!(TokenKind::Semicolon),
+
+            '(' => token!(TokenKind::LParen),
+            ')' => token!(TokenKind::RParen),
+            '{' => token!(TokenKind::LBrace),
+            '}' => token!(TokenKind::RBrace),
+            '[' => token!(TokenKind::LBracket),
+            ']' => token!(TokenKind::RBracket),
+
+            _ => token!(TokenKind::Invalid),
+        };
+
+        let span = Span::new(start, self.cursor);
+        let text = &self.source[start..self.cursor];
+
+        Some(Token::new(kind, text, span))
+    }
+}
+
+impl<'src> Iterator for Lexer<'src> {
+    type Item = Token<'src>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.next_token()
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::frontend::{
+        lexer::Lexer,
+        token::{Span, Token, TokenKind},
+    };
+
+    fn tokenize<'src>(source: &'src str) -> Vec<Token<'src>> {
+        Lexer::new(source).collect()
+    }
+
+    #[test]
+    fn comments() {
+        assert_eq!(
+            tokenize("// this is a comment\nhello"),
+            vec![Token::new(
+                TokenKind::Identifier,
+                "hello",
+                Span::new(21, 26)
+            )]
+        )
+    }
+
+    #[test]
+    fn identifiers() {
+        assert_eq!(
+            tokenize("HELLO _hello _0@"),
+            vec![
+                Token::new(TokenKind::Identifier, "HELLO", Span::new(0, 5)),
+                Token::new(TokenKind::Identifier, "_hello", Span::new(6, 12)),
+                Token::new(TokenKind::Identifier, "_0", Span::new(13, 15)),
+                Token::new(TokenKind::Invalid, "@", Span::new(15, 16)),
+            ]
+        )
+    }
+
+    #[test]
+    fn integer_literals() {
+        assert_eq!(
+            tokenize("0xBEEF 0o777 0b1001 1337"),
+            vec![
+                Token::new(TokenKind::IntegerLit, "0xBEEF", Span::new(0, 6)),
+                Token::new(TokenKind::IntegerLit, "0o777", Span::new(7, 12)),
+                Token::new(TokenKind::IntegerLit, "0b1001", Span::new(13, 19)),
+                Token::new(TokenKind::IntegerLit, "1337", Span::new(20, 24)),
+            ]
+        )
+    }
+
+    #[test]
+    fn boolean_literals() {
+        assert_eq!(
+            tokenize("true false"),
+            vec![
+                Token::new(TokenKind::BooleanLit, "true", Span::new(0, 4)),
+                Token::new(TokenKind::BooleanLit, "false", Span::new(5, 10)),
+            ]
+        );
+    }
+
+    #[test]
+    fn types() {
+        assert_eq!(
+            tokenize("i8 i16 i32 i64 u8 u16 u32 u64 bool"),
+            vec![
+                Token::new(TokenKind::I8, "i8", Span::new(0, 2)),
+                Token::new(TokenKind::I16, "i16", Span::new(3, 6)),
+                Token::new(TokenKind::I32, "i32", Span::new(7, 10)),
+                Token::new(TokenKind::I64, "i64", Span::new(11, 14)),
+                Token::new(TokenKind::U8, "u8", Span::new(15, 17)),
+                Token::new(TokenKind::U16, "u16", Span::new(18, 21)),
+                Token::new(TokenKind::U32, "u32", Span::new(22, 25)),
+                Token::new(TokenKind::U64, "u64", Span::new(26, 29)),
+                Token::new(TokenKind::Bool, "bool", Span::new(30, 34)),
+            ]
+        )
+    }
+
+    #[test]
+    fn operators() {
+        assert_eq!(
+            tokenize("+ - * / %"),
+            vec![
+                Token::new(TokenKind::Plus, "+", Span::new(0, 1)),
+                Token::new(TokenKind::Minus, "-", Span::new(2, 3)),
+                Token::new(TokenKind::Star, "*", Span::new(4, 5)),
+                Token::new(TokenKind::Slash, "/", Span::new(6, 7)),
+                Token::new(TokenKind::Percent, "%", Span::new(8, 9)),
+            ]
+        )
+    }
+
+    #[test]
+    fn punctuation() {
+        assert_eq!(
+            tokenize(". , : ; -> ( ) { } [ ]"),
+            vec![
+                Token::new(TokenKind::Dot, ".", Span::new(0, 1)),
+                Token::new(TokenKind::Comma, ",", Span::new(2, 3)),
+                Token::new(TokenKind::Colon, ":", Span::new(4, 5)),
+                Token::new(TokenKind::Semicolon, ";", Span::new(6, 7)),
+                Token::new(TokenKind::Arrow, "->", Span::new(8, 10)),
+                Token::new(TokenKind::LParen, "(", Span::new(11, 12)),
+                Token::new(TokenKind::RParen, ")", Span::new(13, 14)),
+                Token::new(TokenKind::LBrace, "{", Span::new(15, 16)),
+                Token::new(TokenKind::RBrace, "}", Span::new(17, 18)),
+                Token::new(TokenKind::LBracket, "[", Span::new(19, 20)),
+                Token::new(TokenKind::RBracket, "]", Span::new(21, 22)),
+            ]
+        )
+    }
+}
diff --git a/src/frontend/mod.rs b/src/frontend/mod.rs
index 79c66ba..e12719b 100644
--- a/src/frontend/mod.rs
+++ b/src/frontend/mod.rs
@@ -1 +1,2 @@
+pub mod lexer;
 pub mod token;