diff --git a/src/ast.c b/src/ast.c
index a0ba01c..ccd7d8c 100644
--- a/src/ast.c
+++ b/src/ast.c
@@ -7,9 +7,13 @@
 extern AST* root;
 
 static char* asttype_names[] = {
-    [AST_TYPE_CALL] = "FUNC CALL",     [AST_TYPE_NUM] = "NUMBER",
-    [AST_TYPE_VREF] = "VAR REFERENCE", [AST_TYPE_VDEF] = "VAR DEFINITION",
-    [AST_TYPE_BLOCK] = "BLOCK",        [AST_TYPE_EXC] = "EXCEPTION"
+    [AST_TYPE_CALL] = "FUNC CALL",
+    [AST_TYPE_NUM] = "NUMBER",
+    [AST_TYPE_VREF] = "VAR REFERENCE",
+    [AST_TYPE_VDEF] = "VAR DEFINITION",
+    [AST_TYPE_BLOCK] = "BLOCK",
+    [AST_TYPE_EXC] = "EXCEPTION",
+    [AST_TYPE_FDEF] = "FUNCTION DEFINITION"
 };
 
 AST* ast_init(ASTType type, void* data) {
@@ -30,6 +34,7 @@ void ast_destroy(AST* ast) {
         case AST_TYPE_VREF:  ast_vref_data_destroy(ast->data); break;
         case AST_TYPE_VDEF:  ast_vdef_data_destroy(ast->data); break;
         case AST_TYPE_BLOCK: ast_block_data_destroy(ast->data); break;
+        case AST_TYPE_FDEF:  ast_fdef_data_destroy(ast->data); break;
         default:
             log_dbgf("Unknown ast type %d (max: %d)", ast->type, AST_TYPE_MAX);
     }
@@ -54,6 +59,7 @@ void ast_print_i(AST* ast, int i) {
         case AST_TYPE_VREF:  ast_vref_print(ast->data, i + 2); break;
         case AST_TYPE_VDEF:  ast_vdef_print(ast->data, i + 2); break;
         case AST_TYPE_BLOCK: ast_block_print(ast->data, i + 2); break;
+        case AST_TYPE_FDEF:  ast_fdef_print(ast->data, i + 2); break;
         default:             exit(1);
     }
     INDENT_FIELD_NONL_END;
@@ -209,3 +215,32 @@ void ast_block_print(ASTBlockData* data, int depth) {
 
     INDENT_END;
 }
+
+ASTFDefData*
+ast_fdef_data_init(char* name, size_t argc, AST** argv, AST* body) {
+    ASTFDefData* fdef = malloc(sizeof(ASTFDefData));
+
+    fdef->name = name;
+    fdef->argc = argc;
+    fdef->argv = argv;
+    fdef->body = body;
+
+    return fdef;
+}
+
+void ast_fdef_data_destroy(ASTFDefData* fdef) {
+    free(fdef->name);
+    for (int i = 0; i < fdef->argc; ast_destroy(fdef->argv[i++]));
+    ast_destroy(fdef->body);
+}
+
+void ast_fdef_print(ASTFDefData* fdef, int i) {
+    INDENT_BEGIN(i)
+    INDENT_TITLE("ASTFDefData", fdef);
+    INDENT_FIELD("name", "%s", fdef->name);
+    INDENT_FIELD("argc", "%ld", fdef->argc);
+    INDENT_FIELD_LIST("argv", fdef->argv, fdef->argc, ast_print_i);
+    INDENT_FIELD_EXT_NONL_START("body");
+    ast_print_i(fdef->body, i + 2);
+    INDENT_FIELD_NONL_END;
+}
diff --git a/src/grammar.y b/src/grammar.y
index 5f41658..8e9ea09 100644
--- a/src/grammar.y
+++ b/src/grammar.y
@@ -71,7 +71,6 @@ inputstart:
     }
     ;
 
-
 input:
     inputstart {
         $$ = $1;
@@ -89,8 +88,6 @@ inputend:
     }
     ;
 
-
-
 argstart:
     exp {
         ArgArr* argarr = argarr_init();
@@ -195,4 +192,11 @@ exp:
         strcpy(to, "div");
         $$ = ast_init(AST_TYPE_CALL, ast_call_data_init(to, 2, argv));
     }
+
+    | WORD GROUPS arg GROUPE EQ exp {
+        size_t argc = $3->ln;
+        AST** argv = $3->buf;
+        argarr_destroypsv($3);
+        $$ = ast_init(AST_TYPE_FDEF, ast_fdef_data_init($1, argc, argv, $6));
+    }
 %%
diff --git a/src/include/ast.h b/src/include/ast.h
index 1e33ee2..d0d9bc3 100644
--- a/src/include/ast.h
+++ b/src/include/ast.h
@@ -22,7 +22,8 @@ typedef enum {
     AST_TYPE_VDEF,  // A variable definition.
     AST_TYPE_VREF,  // A variable reference.
     AST_TYPE_BLOCK, // A block of code (scope).
-    AST_TYPE_MAX = AST_TYPE_BLOCK,
+    AST_TYPE_FDEF,  // A function definition.
+    AST_TYPE_MAX = AST_TYPE_FDEF,
 } ASTType;
 
 // An Abstract Syntax Tree.
@@ -52,7 +53,7 @@ void ast_num_print(ASTNumData*, int i);
 
 // An exception.
 typedef struct ASTEXCDATA {
-    char* msg; // The exception message.
+    char* msg;  // The exception message.
     AST* trace; // The previous exception.
 } ASTExcData;
 // Create a new `ASTExecData.
@@ -122,4 +123,19 @@ void ast_block_data_destroy(ASTBlockData* block);
 // Print an `ASTBlockData`.
 void ast_block_print(ASTBlockData*, int i);
 
+typedef struct {
+    char* name;  // Function name.
+    size_t argc; // Argument count.
+    AST** argv;  // Argument vector.
+    AST* body;   // Function body.
+} ASTFDefData;
+
+// Create a new `ASTFDefData`.
+ASTFDefData*
+ast_fdef_data_init(char* name, size_t argc, AST** argv, AST* body);
+// Destroy an `ASTFDefData`.
+void ast_fdef_data_destroy(ASTFDefData* fdef);
+// Print an `ASTFDefData`.
+void ast_fdef_print(ASTFDefData* fdef, int i);
+
 #endif