diff --git a/doc/roza.bnf b/doc/roza.bnf
index 6e3027d..6de25e0 100644
--- a/doc/roza.bnf
+++ b/doc/roza.bnf
@@ -1,2 +1,3 @@
 MOD ::= EXPR*
-EXPR ::= num
+EXPR ::= BUILTIN
+BUILTIN ::= num | bool
diff --git a/lib/compiler.c b/lib/compiler.c
index 7b4335e..e34c7e4 100644
--- a/lib/compiler.c
+++ b/lib/compiler.c
@@ -40,6 +40,15 @@ void compiler_run(compiler_t* compiler, node_t* node)
 
     } break;
 
+    case NODE_BOOL: {
+      int value = strcmp(node->value.data, "true") == 0;
+      value_t* val = tysy_new_bool(compiler->tysy, value);
+      Opcode op = OP_PUSH;
+      param_t param = (param_t) mod_push_new_value(compiler->mod, val);
+
+      mod_push_instr(compiler->mod, op, param);
+    } break;
+
     default: {
       char msg[RZ_STR_LIMIT];
       snprintf(msg, RZ_STR_LIMIT, "Unknown node '%s'",
diff --git a/lib/lexer.c b/lib/lexer.c
index 2ed9c6f..4c7e02e 100644
--- a/lib/lexer.c
+++ b/lib/lexer.c
@@ -36,6 +36,15 @@ node_t* lexer_try_new_next(lexer_t* lexer)
       }
   }
 
+  {
+    node_t* kw = lexer_try_new_keyword(lexer, "true", NODE_BOOL, 1);
+    if (kw) { return kw; }
+  }
+  {
+    node_t* kw = lexer_try_new_keyword(lexer, "false", NODE_BOOL, 1);
+    if (kw) { return kw; }
+  }
+
   // scan num
   {
     size_t cursor = lexer->cursor;
@@ -93,3 +102,72 @@ node_t* lexer_try_new_next(lexer_t* lexer)
 
   return NULL;
 }
+
+NodeType lexer_peek(lexer_t* lexer, int lookahead)
+{
+  assert(lexer);
+  size_t cursor = lexer->cursor;
+  int line = lexer->line;
+
+  NodeType type = 0;
+
+  for (int i=0; i<lookahead; i++)
+    {
+      node_t* node = lexer_try_new_next(lexer);
+
+      if (node)
+        {
+          type = node->type;
+          node_free(node);
+          free(node);
+        }
+    }
+
+  lexer->cursor = cursor;
+  lexer->line = line;
+
+  return type;
+}
+
+node_t* lexer_try_new_keyword(lexer_t* lexer, char* kw,
+                              NodeType type, int has_value)
+{
+  assert(lexer);
+  assert(kw);
+
+  size_t len = strlen(kw);
+  size_t cursor = lexer->cursor;
+
+  if (cursor + len <= strlen(lexer->source))
+    {
+      int ok = 1;
+
+      for (size_t i=cursor; i<cursor + len; i++)
+        {
+          if (lexer->source[i] != kw[i - cursor])
+            {
+              ok = 0;
+              break;
+            }
+        }
+
+      if (ok)
+        {
+          int next_idx = lexer->cursor + len;
+
+          if (next_idx < strlen(lexer->source)
+              && !isspace(lexer->source[next_idx]))
+            {
+              return NULL;
+            }
+
+          node_t* node = malloc(sizeof(node_t));
+          node_init(node, type, has_value ? (char*) kw : "", lexer->line);
+
+          lexer->cursor += len;
+          return node;
+        }
+    }
+
+  return NULL;
+}
diff --git a/lib/lexer.h b/lib/lexer.h
index 179cce6..d8d9503 100644
--- a/lib/lexer.h
+++ b/lib/lexer.h
@@ -16,5 +16,9 @@ void lexer_init(lexer_t* lexer, char const* source, err_t* err);
 void lexer_free(lexer_t* lexer);
 
 node_t* lexer_try_new_next(lexer_t* lexer);
+NodeType lexer_peek(lexer_t* lexer, int lookahead);
+
+node_t* lexer_try_new_keyword(lexer_t* lexer, char* kw,
+                              NodeType type, int has_value);
 
 #endif
diff --git a/lib/node.h b/lib/node.h
index 92e4787..f14eb4c 100644
--- a/lib/node.h
+++ b/lib/node.h
@@ -5,7 +5,8 @@
 
 #define NODE_TYPE(G)                            \
   G(NODE_MOD),                                  \
-    G(NODE_NUM)
+    G(NODE_NUM),                                \
+    G(NODE_BOOL)
 
 RZ_ENUM_H(NodeType, NODE_TYPE);
 
diff --git a/lib/parser.c b/lib/parser.c
index 6583eca..147260e 100644
--- a/lib/parser.c
+++ b/lib/parser.c
@@ -49,13 +49,20 @@ node_t* parser_try_new_mod(parser_t* parser)
 node_t* parser_try_new_expr(parser_t* parser)
 {
   assert(parser);
-  return parser_try_new_num(parser);
+  return parser_try_new_builtin(parser);
 }
 
-node_t* parser_try_new_num(parser_t* parser)
+node_t* parser_try_new_builtin(parser_t* parser)
 {
   assert(parser);
-  return parser_try_new_consume(parser, NODE_NUM);
+  NodeType next = lexer_peek(parser->lexer, 1);
+
+  if (next == NODE_NUM || next == NODE_BOOL)
+    {
+      return parser_try_new_consume(parser, next);
+    }
+
+  return NULL;
 }
 
 node_t* parser_try_new_consume(parser_t* parser, NodeType type)
diff --git a/lib/parser.h b/lib/parser.h
index d259c72..894a393 100644
--- a/lib/parser.h
+++ b/lib/parser.h
@@ -16,7 +16,7 @@ void parser_free(parser_t* parser);
 node_t* parser_try_new_tree(parser_t* parser);
 node_t* parser_try_new_mod(parser_t* parser);
 node_t* parser_try_new_expr(parser_t* parser);
-node_t* parser_try_new_num(parser_t* parser);
+node_t* parser_try_new_builtin(parser_t* parser);
 
 node_t* parser_try_new_consume(parser_t* parser, NodeType type);
 
diff --git a/lib/type.h b/lib/type.h
index f0e4331..a25e79e 100644
--- a/lib/type.h
+++ b/lib/type.h
@@ -4,7 +4,8 @@
 #include "commons.h"
 
 #define TYPE_KIND(G)                            \
-  G(TYPE_NUM)
+  G(TYPE_NUM),                                  \
+    G(TYPE_BOOL)
 
 RZ_ENUM_H(TypeKind, TYPE_KIND);
 
diff --git a/lib/tysy.c b/lib/tysy.c
index 5984655..d05a597 100644
--- a/lib/tysy.c
+++ b/lib/tysy.c
@@ -7,9 +7,16 @@ void tysy_init(tysy_t* tysy)
 
   // num
   {
-    type_t* num = malloc(sizeof(type_t));
-    type_init(num, TYPE_NUM);
-    tysy_register_new_type(tysy, "num", num);
+    type_t* ty = malloc(sizeof(type_t));
+    type_init(ty, TYPE_NUM);
+    tysy_register_new_type(tysy, "num", ty);
+  }
+
+  // bool
+  {
+    type_t* ty = malloc(sizeof(type_t));
+    type_init(ty, TYPE_BOOL);
+    tysy_register_new_type(tysy, "bool", ty);
   }
 }
 
@@ -65,3 +72,14 @@ value_t* tysy_new_num(tysy_t* tysy, double value)
 
   return val;
 }
+
+value_t* tysy_new_bool(tysy_t* tysy, int value)
+{
+  assert(tysy);
+
+  value_t* val = malloc(sizeof(value_t));
+  value_init(val, tysy_try_find_type(tysy, "bool"));
+  val->value.bool = value;
+
+  return val;
+}
diff --git a/lib/tysy.h b/lib/tysy.h
index 1719120..e947fa7 100644
--- a/lib/tysy.h
+++ b/lib/tysy.h
@@ -17,5 +17,6 @@ void tysy_register_new_type(tysy_t* tysy, char* name, type_t* type);
 type_t* tysy_try_find_type(tysy_t* tysy, char* name);
 
 value_t* tysy_new_num(tysy_t* tysy, double value);
+value_t* tysy_new_bool(tysy_t* tysy, int value);
 
 #endif
diff --git a/lib/value.c b/lib/value.c
index 44bb89a..2c8ed43 100644
--- a/lib/value.c
+++ b/lib/value.c
@@ -26,6 +26,10 @@ int value_eq(value_t* value, value_t* rhs)
       return value->value.num == rhs->value.num;
     } break;
 
+    case TYPE_BOOL: {
+      return value->value.bool == rhs->value.bool;
+    } break;
+
     default: {
       fprintf(stderr, "Cannot compare value of type '%s'.\n",
               TypeKindStr[value->type->kind]);
@@ -47,6 +51,14 @@ size_t value_str(value_t* value, char* buffer, size_t size)
       return snprintf(buffer, size, "%lf", value->value.num);
       break;
 
-    default: return 0;
+    case TYPE_BOOL:
+      return snprintf(buffer, size, "%s", value->value.bool ? "true" : "false");
+      break;
+
+    default: {
+      fprintf(stderr, "Cannot stringify unknown value of type '%s'.\n",
+              TypeKindStr[value->type->kind]);
+      abort();
+    };
     }
 }
diff --git a/lib/value.h b/lib/value.h
index bc96092..5481b09 100644
--- a/lib/value.h
+++ b/lib/value.h
@@ -8,6 +8,7 @@ typedef struct {
 
   union {
     double num;
+    int bool;
   } value;
 
 } value_t;
diff --git a/tests/units/lexer.c b/tests/units/lexer.c
index 458eab6..c7a75e9 100644
--- a/tests/units/lexer.c
+++ b/tests/units/lexer.c
@@ -81,3 +81,15 @@ Test(lexer, num) {
   test_lexer_ko("..2");
   test_lexer_ko("2..");
 }
+
+Test(lexer, bool) {
+  test_lexer_ko("true3");
+  test_lexer_ko("1true");
+  test_lexer_ko("afalse");
+  test_lexer_ko("falset");
+
+  test_lexer("true 3 false", 3,
+             "BOOL[true]",
+             "NUM[3]",
+             "BOOL[false]");
+}
diff --git a/tests/units/parser.c b/tests/units/parser.c
index 967f9c4..52c39a9 100644
--- a/tests/units/parser.c
+++ b/tests/units/parser.c
@@ -58,3 +58,7 @@ Test(parser, num) {
 
   test_parser_ko(" § ");
 }
+
+Test(parser, bool) {
+  test_parser_ok("MOD(BOOL[true],BOOL[false])", "true false");
+}