diff --git a/include/tree_sitter/parser.h b/include/tree_sitter/parser.h
index a335dd6d..e099fd7f 100644
--- a/include/tree_sitter/parser.h
+++ b/include/tree_sitter/parser.h
@@ -65,6 +65,7 @@ typedef union {
 typedef struct TSLanguage {
   uint32_t symbol_count;
   uint32_t token_count;
+  uint32_t external_token_count;
   const char **symbol_names;
   const TSSymbolMetadata *symbol_metadata;
   const unsigned short *parse_table;
@@ -75,7 +76,7 @@ typedef struct TSLanguage {
   const bool *external_token_lists;
   struct {
     void * (*create)();
-    bool (*scan)(TSLexer *, const bool *symbol_whitelist);
+    bool (*scan)(void *, TSLexer *, const bool *symbol_whitelist);
     void (*destroy)(void *);
   } external_scanner;
 } TSLanguage;
@@ -158,7 +159,6 @@ typedef struct TSLanguage {
     { .type = TSParseActionTypeAccept } \
   }
 
-
 #define GET_LANGUAGE(...)                                          \
   static TSLanguage language = {                                   \
     .symbol_count = SYMBOL_COUNT,                                  \
@@ -169,6 +169,7 @@ typedef struct TSLanguage {
     .lex_modes = ts_lex_modes,                                     \
     .symbol_names = ts_symbol_names,                               \
     .lex_fn = ts_lex,                                              \
+    .external_token_count = EXTERNAL_TOKEN_COUNT,                  \
     .external_token_lists = (const bool *)ts_external_token_lists, \
     .external_token_symbol_map = ts_external_token_symbol_map,     \
     .external_scanner = {__VA_ARGS__}                              \
diff --git a/spec/compiler/prepare_grammar/extract_tokens_spec.cc b/spec/compiler/prepare_grammar/extract_tokens_spec.cc
index 577dead1..613d31cc 100644
--- a/spec/compiler/prepare_grammar/extract_tokens_spec.cc
+++ b/spec/compiler/prepare_grammar/extract_tokens_spec.cc
@@ -5,6 +5,7 @@
 #include "compiler/prepare_grammar/extract_tokens.h"
 #include "helpers/rule_helpers.h"
 #include "helpers/equals_pointer.h"
+#include "helpers/stream_methods.h"
 
 START_TEST
 
@@ -211,9 +212,10 @@ describe("extract_tokens", []() {
       }, { choice({ i_sym(1), blank() }) }, {}});
 
       AssertThat(get<2>(result), !Equals(CompileError::none()));
-      AssertThat(get<2>(result), Equals(
-        CompileError(TSCompileErrorTypeInvalidUbiquitousToken,
-                         "Not a token: (choice (sym 1) (blank))")));
+      AssertThat(get<2>(result), Equals(CompileError(
+        TSCompileErrorTypeInvalidUbiquitousToken,
+        "Not a token: (choice (non-terminal 1) (blank))"
+      )));
     });
   });
 });
diff --git a/spec/fixtures/external_scanners/external_scan.c b/spec/fixtures/external_scanners/external_scan.c
index 7abab3ae..41ef3706 100644
--- a/spec/fixtures/external_scanners/external_scan.c
+++ b/spec/fixtures/external_scanners/external_scan.c
@@ -1,13 +1,108 @@
 #include <stdbool.h>
+#include <tree_sitter/parser.h>
+
+enum {
+  percent_string,
+  percent_string_start,
+  percent_string_end
+};
+
+typedef struct {
+  int32_t open_delimiter;
+  int32_t close_delimiter;
+  uint32_t depth;
+} Scanner;
 
 void *ts_language_external_scanner_example_external_scanner_create() {
-  puts("HELLO FROM EXTERNAL SCANNER");
-  return 0;
+  Scanner *scanner = malloc(sizeof(Scanner));
+  *scanner = (Scanner){
+    .open_delimiter = 0,
+    .close_delimiter = 0,
+    .depth = 0
+  };
+  return scanner;
 }
 
-bool ts_language_external_scanner_example_external_scanner_scan() {
-  return true;
+bool ts_language_external_scanner_example_external_scanner_scan(
+  void *payload, TSLexer *lexer, const bool *whitelist) {
+  Scanner *scanner = payload;
+
+  if (whitelist[percent_string]) {
+    while (lexer->lookahead == ' ' ||
+           lexer->lookahead == '\t' ||
+           lexer->lookahead == '\n') {
+      lexer->advance(lexer, 0, true);
+    }
+
+    if (lexer->lookahead != '%') return false;
+    lexer->advance(lexer, 0, false);
+
+    switch (lexer->lookahead) {
+      case '(':
+        scanner->open_delimiter = '(';
+        scanner->close_delimiter = ')';
+        scanner->depth = 1;
+        break;
+      case '[':
+        scanner->open_delimiter = '[';
+        scanner->close_delimiter = ']';
+        scanner->depth = 1;
+        break;
+      case '{':
+        scanner->open_delimiter = '{';
+        scanner->close_delimiter = '}';
+        scanner->depth = 1;
+        break;
+      default:
+        return false;
+    }
+
+    lexer->advance(lexer, 0, false);
+
+    for (;;) {
+      if (scanner->depth == 0) {
+        lexer->result_symbol = percent_string;
+        return true;
+      }
+
+      if (lexer->lookahead == scanner->open_delimiter) {
+        scanner->depth++;
+      } else if (lexer->lookahead == scanner->close_delimiter) {
+        scanner->depth--;
+      } else if (lexer->lookahead == '#') {
+        lexer->advance(lexer, 0, false);
+        if (lexer->lookahead == '{') {
+          lexer->advance(lexer, 0, false);
+          lexer->result_symbol = percent_string_start;
+          return true;
+        }
+      }
+
+      lexer->advance(lexer, 0, false);
+    }
+  } else if (whitelist[percent_string_end]) {
+    if (lexer->lookahead != '}') return false;
+    lexer->advance(lexer, 0, false);
+
+    for (;;) {
+      if (scanner->depth == 0) {
+        lexer->result_symbol = percent_string_end;
+        return true;
+      }
+
+      if (lexer->lookahead == scanner->open_delimiter) {
+        scanner->depth++;
+      } else if (lexer->lookahead == scanner->close_delimiter) {
+        scanner->depth--;
+      }
+
+      lexer->advance(lexer, 0, false);
+    }
+  }
+
+  return false;
 }
 
-void ts_language_external_scanner_example_external_scanner_destroy() {
+void ts_language_external_scanner_example_external_scanner_destroy(void *payload) {
+  free(payload);
 }
diff --git a/spec/integration/compile_grammar_spec.cc b/spec/integration/compile_grammar_spec.cc
index 21307c89..934b428c 100644
--- a/spec/integration/compile_grammar_spec.cc
+++ b/spec/integration/compile_grammar_spec.cc
@@ -562,7 +562,11 @@ describe("compile_grammar", []() {
         "spec/fixtures/external_scanners/external_scan.c"
       ));
 
-      ts_document_set_input_string(document, "%|hi|");
+      ts_document_set_input_string(document, "%(sup (external) scanner?)");
+      ts_document_parse(document);
+      assert_root_node("(string)");
+
+      ts_document_set_input_string(document, "%{sup {} external {} scanner?}");
       ts_document_parse(document);
       assert_root_node("(string)");
 
diff --git a/src/compiler/generate_code/c_code.cc b/src/compiler/generate_code/c_code.cc
index a5a9c17a..36bd7cab 100644
--- a/src/compiler/generate_code/c_code.cc
+++ b/src/compiler/generate_code/c_code.cc
@@ -128,7 +128,7 @@ class CCodeGenerator {
   void add_stats() {
     line("#define STATE_COUNT " + to_string(parse_table.states.size()));
     line("#define SYMBOL_COUNT " + to_string(parse_table.symbols.size()));
-    line("#define TOKEN_COUNT " + to_string(lexical_grammar.variables.size() + 1));
+    line("#define TOKEN_COUNT " + to_string(lexical_grammar.variables.size() + 1 + syntax_grammar.external_tokens.size()));
     line("#define EXTERNAL_TOKEN_COUNT " + to_string(syntax_grammar.external_tokens.size()));
     line();
   }
@@ -327,7 +327,7 @@ class CCodeGenerator {
       string external_scanner_name = "ts_language_" + name + "_external_scanner";
 
       line("void *" + external_scanner_name + "_create();");
-      line("bool " + external_scanner_name + "_scan();");
+      line("bool " + external_scanner_name + "_scan(void *, TSLexer *, const bool *);");
       line("void " + external_scanner_name + "_destroy();");
       line();
 
diff --git a/src/compiler/rules/symbol.h b/src/compiler/rules/symbol.h
index 46272dc5..4aacf1b2 100644
--- a/src/compiler/rules/symbol.h
+++ b/src/compiler/rules/symbol.h
@@ -13,8 +13,8 @@ class Symbol : public Rule {
 
   typedef enum {
     Terminal,
-    NonTerminal,
     External,
+    NonTerminal,
   } Type;
 
   Symbol(Index index, Type type);
diff --git a/src/runtime/document.c b/src/runtime/document.c
index 65f9e435..c68d8c62 100644
--- a/src/runtime/document.c
+++ b/src/runtime/document.c
@@ -37,7 +37,7 @@ const TSLanguage *ts_document_language(TSDocument *self) {
 
 void ts_document_set_language(TSDocument *self, const TSLanguage *language) {
   ts_document_invalidate(self);
-  self->parser.language = language;
+  parser_set_language(&self->parser, language);
   if (self->tree) {
     ts_tree_release(self->tree);
     self->tree = NULL;
diff --git a/src/runtime/language.h b/src/runtime/language.h
index a4f44b11..5a2693db 100644
--- a/src/runtime/language.h
+++ b/src/runtime/language.h
@@ -49,6 +49,12 @@ static inline TSStateId ts_language_next_state(const TSLanguage *self,
   }
 }
 
+static inline const bool *
+ts_language_enabled_external_tokens(const TSLanguage *self,
+                                    unsigned external_scanner_state) {
+  return self->external_token_lists + self->external_token_count * external_scanner_state;
+}
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/runtime/lexer.c b/src/runtime/lexer.c
index 32910935..77d76ec6 100644
--- a/src/runtime/lexer.c
+++ b/src/runtime/lexer.c
@@ -123,9 +123,7 @@ void ts_lexer_reset(Lexer *self, Length position) {
   return;
 }
 
-void ts_lexer_start(Lexer *self, TSStateId lex_state) {
-  LOG("start_lex state:%d, pos:%u", lex_state, self->current_position.chars);
-
+void ts_lexer_start(Lexer *self) {
   self->token_start_position = self->current_position;
   self->data.result_symbol = 0;
 
diff --git a/src/runtime/lexer.h b/src/runtime/lexer.h
index 1b047e5b..682c3f93 100644
--- a/src/runtime/lexer.h
+++ b/src/runtime/lexer.h
@@ -30,7 +30,7 @@ typedef struct {
 void ts_lexer_init(Lexer *);
 void ts_lexer_set_input(Lexer *, TSInput);
 void ts_lexer_reset(Lexer *, Length);
-void ts_lexer_start(Lexer *, TSStateId);
+void ts_lexer_start(Lexer *);
 
 #ifdef __cplusplus
 }
diff --git a/src/runtime/parser.c b/src/runtime/parser.c
index c37b7871..997103c8 100644
--- a/src/runtime/parser.c
+++ b/src/runtime/parser.c
@@ -209,23 +209,43 @@ static bool parser__condense_stack(Parser *self) {
 }
 
 static Tree *parser__lex(Parser *self, TSStateId parse_state) {
+  Length start_position = self->lexer.current_position;
+  ts_lexer_start(&self->lexer);
+
+  TSLexMode lex_mode = self->language->lex_modes[parse_state];
+  if (lex_mode.external_tokens) {
+    const bool *external_tokens = ts_language_enabled_external_tokens(self->language, lex_mode.external_tokens);
+    if (self->language->external_scanner.scan(
+      self->external_scanner_payload,
+      &self->lexer.data,
+      external_tokens
+    )) {
+      TSSymbol symbol = self->language->external_token_symbol_map[self->lexer.data.result_symbol];
+      Length padding = length_sub(self->lexer.token_start_position, start_position);
+      Length size = length_sub(self->lexer.current_position, self->lexer.token_start_position);
+      TSSymbolMetadata metadata = ts_language_symbol_metadata(self->language, symbol);
+      Tree *result = ts_tree_make_leaf(symbol, padding, size, metadata);
+      result->parse_state = parse_state;
+      return result;
+    } else {
+      ts_lexer_reset(&self->lexer, start_position);
+    }
+  }
+
   TSStateId start_state = self->language->lex_modes[parse_state].lex_state;
   TSStateId current_state = start_state;
-  Length start_position = self->lexer.current_position;
   LOG("lex state:%d", start_state);
 
   bool skipped_error = false;
   int32_t first_error_character = 0;
   Length error_start_position, error_end_position;
 
-  ts_lexer_start(&self->lexer, start_state);
-
   while (!self->language->lex_fn(&self->lexer.data, current_state)) {
     if (current_state != ERROR_STATE) {
       LOG("retry_in_error_mode");
       current_state = ERROR_STATE;
       ts_lexer_reset(&self->lexer, start_position);
-      ts_lexer_start(&self->lexer, current_state);
+      ts_lexer_start(&self->lexer);
       continue;
     }
 
@@ -247,7 +267,6 @@ static Tree *parser__lex(Parser *self, TSStateId parse_state) {
   }
 
   Tree *result;
-
   if (skipped_error) {
     Length padding = length_sub(error_start_position, start_position);
     Length size = length_sub(error_end_position, error_start_position);
@@ -255,18 +274,12 @@ static Tree *parser__lex(Parser *self, TSStateId parse_state) {
     result = ts_tree_make_error(size, padding, first_error_character);
   } else {
     TSSymbol symbol = self->lexer.data.result_symbol;
-    Length padding =
-      length_sub(self->lexer.token_start_position, start_position);
-    Length size = length_sub(self->lexer.current_position,
-                                  self->lexer.token_start_position);
-    result =
-      ts_tree_make_leaf(symbol, padding, size,
-                        ts_language_symbol_metadata(self->language, symbol));
+    Length padding = length_sub(self->lexer.token_start_position, start_position);
+    Length size = length_sub(self->lexer.current_position, self->lexer.token_start_position);
+    TSSymbolMetadata metadata = ts_language_symbol_metadata(self->language, symbol);
+    result = ts_tree_make_leaf(symbol, padding, size, metadata);
   }
 
-  if (!result)
-    return NULL;
-
   result->parse_state = parse_state;
   result->first_leaf.lex_state = start_state;
   return result;
@@ -1106,6 +1119,15 @@ bool parser_init(Parser *self) {
   return true;
 }
 
+void parser_set_language(Parser *self, const TSLanguage *language) {
+  self->language = language;
+  if (language->external_scanner.create) {
+    self->external_scanner_payload = language->external_scanner.create();
+  } else {
+    self->external_scanner_payload = NULL;
+  }
+}
+
 void parser_destroy(Parser *self) {
   if (self->stack)
     ts_stack_delete(self->stack);
diff --git a/src/runtime/parser.h b/src/runtime/parser.h
index 41512e12..54c041b3 100644
--- a/src/runtime/parser.h
+++ b/src/runtime/parser.h
@@ -29,11 +29,13 @@ typedef struct {
   ReusableNode reusable_node;
   TreePath tree_path1;
   TreePath tree_path2;
+  void *external_scanner_payload;
 } Parser;
 
 bool parser_init(Parser *);
 void parser_destroy(Parser *);
 Tree *parser_parse(Parser *, TSInput, Tree *);
+void parser_set_language(Parser *, const TSLanguage *);
 
 #ifdef __cplusplus
 }