diff --git a/src/main/kotlin/lumen/lexer/Lexer.kt b/src/main/kotlin/lumen/lexer/Lexer.kt
new file mode 100644
index 0000000..b3c36c6
--- /dev/null
+++ b/src/main/kotlin/lumen/lexer/Lexer.kt
@@ -0,0 +1,200 @@
+package lumen.lexer
+
+import lumen.LexError
+
+private val keywords = mapOf(
+    "let" to TokenType.Let,
+    "var" to TokenType.Var,
+    "fn" to TokenType.Fn,
+    "return" to TokenType.Return,
+    "if" to TokenType.If,
+    "else" to TokenType.Else,
+    "while" to TokenType.While,
+    "for" to TokenType.For,
+    "in" to TokenType.In,
+    "true" to TokenType.True,
+    "false" to TokenType.False,
+    "nil" to TokenType.Nil,
+    "and" to TokenType.And,
+    "or" to TokenType.Or,
+    "not" to TokenType.Not,
+    "import" to TokenType.Import
+)
+
+class Lexer(private val source: String) {
+    private val tokens = mutableListOf<Token>()
+    private var start = 0
+    private var current = 0
+    private var line = 1
+    private var lineStart = 0
+    private var groupingDepth = 0
+
+    fun scan(): List<Token> {
+        while (!atEnd()) {
+            start = current
+            scanToken()
+        }
+        tokens.add(Token(TokenType.Eof, "", line, current - lineStart + 1))
+        return tokens
+    }
+
+    private fun scanToken() {
+        val c = advance()
+        when (c) {
+            ' ', '\t', '\r' -> {}
+            '\n' -> {
+                if (groupingDepth == 0) add(TokenType.Newline)
+                line++
+                lineStart = current
+            }
+            '(' -> {
+                groupingDepth++
+                add(TokenType.LeftParen)
+            }
+            ')' -> {
+                if (groupingDepth > 0) groupingDepth--
+                add(TokenType.RightParen)
+            }
+            '[' -> {
+                groupingDepth++
+                add(TokenType.LeftBracket)
+            }
+            ']' -> {
+                if (groupingDepth > 0) groupingDepth--
+                add(TokenType.RightBracket)
+            }
+            '{' -> add(TokenType.LeftBrace)
+            '}' -> add(TokenType.RightBrace)
+            ',' -> add(TokenType.Comma)
+            ':' -> add(TokenType.Colon)
+            ';' -> add(TokenType.Semicolon)
+            '.' -> add(TokenType.Dot)
+            '+' -> add(TokenType.Plus)
+            '-' -> add(TokenType.Minus)
+            '*' -> add(TokenType.Star)
+            '%' -> add(TokenType.Percent)
+            '/' -> when {
+                match('/') -> lineComment()
+                match('*') -> blockComment()
+                else -> add(TokenType.Slash)
+            }
+            '=' -> add(if (match('=')) TokenType.EqualEqual else TokenType.Assign)
+            '!' -> {
+                if (match('=')) add(TokenType.BangEqual)
+                else throw LexError("unexpected character '!'", line, columnOf(start))
+            }
+            '<' -> add(if (match('=')) TokenType.LessEqual else TokenType.Less)
+            '>' -> add(if (match('=')) TokenType.GreaterEqual else TokenType.Greater)
+            '"' -> string()
+            else -> when {
+                c.isDigit() -> number()
+                isIdentStart(c) -> identifier()
+                else -> throw LexError("unexpected character '$c'", line, columnOf(start))
+            }
+        }
+    }
+
+    private fun lineComment() {
+        while (!atEnd() && peek() != '\n') advance()
+    }
+
+    private fun blockComment() {
+        var depth = 1
+        while (!atEnd() && depth > 0) {
+            val c = advance()
+            when {
+                c == '\n' -> {
+                    line++
+                    lineStart = current
+                }
+                c == '/' && peek() == '*' -> {
+                    advance()
+                    depth++
+                }
+                c == '*' && peek() == '/' -> {
+                    advance()
+                    depth--
+                }
+            }
+        }
+        if (depth > 0) throw LexError("unterminated block comment", line, columnOf(start))
+    }
+
+    private fun string() {
+        val builder = StringBuilder()
+        while (!atEnd() && peek() != '"') {
+            val c = advance()
+            when {
+                c == '\n' -> {
+                    builder.append('\n')
+                    line++
+                    lineStart = current
+                }
+                c == '\\' -> builder.append(escape())
+                else -> builder.append(c)
+            }
+        }
+        if (atEnd()) throw LexError("unterminated string", line, columnOf(start))
+        advance()
+        tokens.add(Token(TokenType.StringLiteral, builder.toString(), line, columnOf(start), stringValue = builder.toString()))
+    }
+
+    private fun escape(): Char {
+        if (atEnd()) throw LexError("unterminated escape sequence", line, columnOf(start))
+        return when (val e = advance()) {
+            'n' -> '\n'
+            't' -> '\t'
+            'r' -> '\r'
+            '"' -> '"'
+            '\\' -> '\\'
+            '0' -> ' '
+            else -> throw LexError("invalid escape '\\$e'", line, columnOf(current - 2))
+        }
+    }
+
+    private fun number() {
+        while (peek().isDigit()) advance()
+        var isDouble = false
+        if (peek() == '.' && peekNext().isDigit()) {
+            isDouble = true
+            advance()
+            while (peek().isDigit()) advance()
+        }
+        val text = source.substring(start, current)
+        if (isDouble) {
+            tokens.add(Token(TokenType.DoubleLiteral, text, line, columnOf(start), doubleValue = text.toDouble()))
+        } else {
+            tokens.add(Token(TokenType.IntLiteral, text, line, columnOf(start), intValue = text.toLong()))
+        }
+    }
+
+    private fun identifier() {
+        while (isIdentPart(peek())) advance()
+        val text = source.substring(start, current)
+        add(keywords[text] ?: TokenType.Identifier)
+    }
+
+    private fun isIdentStart(c: Char) = c.isLetter() || c == '_'
+
+    private fun isIdentPart(c: Char) = c.isLetterOrDigit() || c == '_'
+
+    private fun add(type: TokenType) {
+        tokens.add(Token(type, source.substring(start, current), line, columnOf(start)))
+    }
+
+    private fun columnOf(index: Int): Int = index - lineStart + 1
+
+    private fun advance(): Char = source[current++]
+
+    private fun match(expected: Char): Boolean {
+        if (atEnd() || source[current] != expected) return false
+        current++
+        return true
+    }
+
+    private fun peek(): Char = if (atEnd()) ' ' else source[current]
+
+    private fun peekNext(): Char = if (current + 1 >= source.length) ' ' else source[current + 1]
+
+    private fun atEnd(): Boolean = current >= source.length
+}