diff --git a/clox/src/scanner.c b/clox/src/scanner.c
index e125a99..15d6484 100644
--- a/clox/src/scanner.c
+++ b/clox/src/scanner.c
@@ -18,8 +18,32 @@ void initScanner(const char *source) {
   scanner.line = 1;
 }
 
+static bool isDigit(char c) { return c >= '0' && c <= '9'; }
+
 static bool isAtEnd() { return *scanner.current == '\0'; }
 
+static char advance() {
+  scanner.current++;
+  return scanner.current[-1];
+}
+
+static char peek() { return *scanner.current; }
+
+static char peekNext() {
+  if (isAtEnd())
+    return '\0';
+  return scanner.current[1];
+}
+
+static bool match(char expected) {
+  if (isAtEnd())
+    return false;
+  if (*scanner.current != expected)
+    return false;
+  scanner.current++;
+  return true;
+}
+
 static Token makeToken(TokenType type) {
   Token token;
   token.type = type;
@@ -38,11 +62,111 @@ static Token errorToken(const char *message) {
   return token;
 }
 
+static void skipWhitespace() {
+  for (;;) {
+    char c = peek();
+    switch (c) {
+    case ' ':
+    case '\r':
+    case '\t':
+      advance();
+      break;
+    case '\n':
+      scanner.line++;
+      advance();
+      break;
+    case '/':
+      if (peekNext() == '/') {
+        // A comment goes until the end of the line.
+        while (peek() != '\n' && !isAtEnd())
+          advance();
+      } else {
+        return;
+      }
+      break;
+
+    default:
+      return;
+    }
+  }
+}
+
+static Token number() {
+  while (isDigit(peek()))
+    advance();
+
+  // Look for a fractional part.
+  if (peek() == '.' && isDigit(peekNext())) {
+    // Consume the ".";
+    advance();
+
+    while (isDigit(peek()))
+      advance();
+  }
+
+  return makeToken(TOKEN_NUMBER);
+}
+
+static Token string() {
+  while (peek() != '"' && !isAtEnd()) {
+    if (peek() == '\n')
+      scanner.line++;
+    advance();
+  }
+
+  if (isAtEnd())
+    return errorToken("Unterminated string.");
+
+  // The closing quote.
+  advance();
+  return makeToken(TOKEN_STRING);
+}
+
 Token scanToken() {
+  skipWhitespace();
   scanner.start = scanner.current;
 
   if (isAtEnd())
     return makeToken(TOKEN_EOF);
 
+  char c = advance();
+  if (isDigit(c))
+    return number();
+
+  switch (c) {
+  case '(':
+    return makeToken(TOKEN_LEFT_PAREN);
+  case ')':
+    return makeToken(TOKEN_RIGHT_PAREN);
+  case '{':
+    return makeToken(TOKEN_LEFT_BRACE);
+  case '}':
+    return makeToken(TOKEN_RIGHT_BRACE);
+  case ';':
+    return makeToken(TOKEN_SEMICOLON);
+  case ',':
+    return makeToken(TOKEN_COMMA);
+  case '.':
+    return makeToken(TOKEN_DOT);
+  case '-':
+    return makeToken(TOKEN_MINUS);
+  case '+':
+    return makeToken(TOKEN_PLUS);
+  case '/':
+    return makeToken(TOKEN_SLASH);
+  case '*':
+    return makeToken(TOKEN_STAR);
+  case '!':
+    return makeToken(match('=') ? TOKEN_BANG_EQUAL : TOKEN_BANG);
+  case '=':
+    return makeToken(match('=') ? TOKEN_EQUAL_EQUAL : TOKEN_EQUAL);
+  case '<':
+    return makeToken(match('=') ? TOKEN_LESS_EQUAL : TOKEN_LESS);
+  case '>':
+    return makeToken(match('=') ? TOKEN_GREATER_EQUAL : TOKEN_GREATER);
+  case '"':
+    return string();
+  }
+
   return errorToken("Unexpected character.");
 }