diff --git a/gcc/Make-lang.in b/gcc/Make-lang.in
index ae52ac1..5d4af02 100644
--- a/gcc/Make-lang.in
+++ b/gcc/Make-lang.in
@@ -24,6 +24,7 @@ elna_OBJS = \
 	elna/elna-generic.o \
 	elna/elna-convert.o \
 	elna/elna-diagnostic.o \
+	elna/elna-tree.o \
 	elna/ast.o \
 	elna/driver.o \
 	elna/lexer.o \
diff --git a/gcc/elna-diagnostic.cc b/gcc/elna-diagnostic.cc
index c41592a..e388310 100644
--- a/gcc/elna-diagnostic.cc
+++ b/gcc/elna-diagnostic.cc
@@ -1,4 +1,5 @@
 #include "elna/gcc/elna-diagnostic.h"
+#include "elna/gcc/elna-tree.h"
 
 namespace elna
 {
@@ -23,6 +24,10 @@ namespace gcc
         {
             return "Boolean";
         }
+        else if (type == enumeral_node)
+        {
+            return "Enum";
+        }
         else
         {
             return "<<unknown-type>>";
diff --git a/gcc/elna-generic.cc b/gcc/elna-generic.cc
index ca8bc87..ac24dcd 100644
--- a/gcc/elna-generic.cc
+++ b/gcc/elna-generic.cc
@@ -1,4 +1,5 @@
 #include "elna/gcc/elna-generic.h"
+#include "elna/gcc/elna-tree.h"
 #include "elna/gcc/elna-diagnostic.h"
 
 #include "input.h"
@@ -97,12 +98,12 @@ namespace gcc
 
     void generic_visitor::visit(source::integer_literal *literal)
     {
-        current_expression = build_int_cst_type(integer_type_node, literal->number());
+        this->current_expression = build_int_cst_type(integer_type_node, literal->number());
     }
 
     void generic_visitor::visit(source::boolean_literal *literal)
     {
-        current_expression = build_int_cst_type(boolean_type_node, literal->boolean());
+        this->current_expression = build_int_cst_type(boolean_type_node, literal->boolean());
     }
 
     void generic_visitor::visit(source::binary_expression *expression)
@@ -179,6 +180,34 @@ namespace gcc
                 operator_code, integer_type_node, left, right);
     }
 
+    void generic_visitor::visit(source::constant_definition *definition)
+    {
+        location_t definition_location = get_location(&definition->position());
+        tree definition_tree = build_decl(definition_location, CONST_DECL,
+                get_identifier(definition->identifier().c_str()), integer_type_node);
+        auto result = this->symbol_map.insert({ definition->identifier(), definition_tree });
+
+        if (result.second)
+        {
+            definition->body().accept(this);
+
+            DECL_INITIAL(definition_tree) = build_int_cst_type(integer_type_node, definition->body().number());
+            TREE_CONSTANT(definition_tree) = 1;
+            TREE_READONLY(definition_tree) = 1;
+
+            auto declaration_statement = build1_loc(definition_location, DECL_EXPR,
+                    void_type_node, definition_tree);
+            append_to_statement_list(declaration_statement, &this->current_statements);
+        }
+        else
+        {
+            error_at(definition_location,
+                    "variable '%s' already declared in this scope",
+                    definition->identifier().c_str());
+        }
+        this->current_expression = NULL_TREE;
+    }
+
     void generic_visitor::visit(source::declaration *declaration)
     {
         if (declaration->type().base() != "Int" && declaration->type().base() != "Bool")
@@ -236,9 +265,16 @@ namespace gcc
         }
         statement->rvalue().accept(this);
 
+        if (TREE_CODE(lvalue->second) == CONST_DECL)
+        {
+            error_at(statement_location, "cannot modify constant '%s'",
+                    statement->lvalue().c_str());
+            this->current_expression = error_mark_node;
+            return;
+        }
         if (TREE_TYPE(this->current_expression) != TREE_TYPE(lvalue->second))
         {
-            error_at(get_location(&statement->position()),
+            error_at(statement_location,
                     "cannot assign value of type %s to variable '%s' of type %s",
                     print_type(TREE_TYPE(this->current_expression)),
                     statement->lvalue().c_str(),
diff --git a/gcc/elna-tree.cc b/gcc/elna-tree.cc
new file mode 100644
index 0000000..b151e82
--- /dev/null
+++ b/gcc/elna-tree.cc
@@ -0,0 +1,3 @@
+#include "elna/gcc/elna-tree.h"
+
+tree elna_global_trees[ELNA_TI_MAX];
diff --git a/gcc/elna1.cc b/gcc/elna1.cc
index 574b196..b93b322 100644
--- a/gcc/elna1.cc
+++ b/gcc/elna1.cc
@@ -16,6 +16,7 @@
 
 #include <fstream>
 #include <elna/source/driver.h>
+#include "elna/gcc/elna-tree.h"
 #include "elna/gcc/elna-generic.h"
 #include "elna/gcc/elna-diagnostic.h"
 #include "parser.hh"
@@ -64,17 +65,18 @@ struct GTY (()) language_function
 
 static bool elna_langhook_init(void)
 {
-    build_common_tree_nodes (false);
+    build_common_tree_nodes(false);
+    enumeral_node = make_node(ENUMERAL_TYPE);
 
     /* I don't know why this has to be done explicitly.  */
-    void_list_node = build_tree_list (NULL_TREE, void_type_node);
+    void_list_node = build_tree_list(NULL_TREE, void_type_node);
 
-    build_common_builtin_nodes ();
+    build_common_builtin_nodes();
 
     return true;
 }
 
-static void elna_parse_file (const char *filename)
+static void elna_parse_file(const char *filename)
 {
     std::ifstream file{ filename, std::ios::in };
 
@@ -106,12 +108,11 @@ static void elna_parse_file (const char *filename)
     linemap_add(line_table, LC_LEAVE, 0, NULL, 0);
 }
 
-static void
-elna_langhook_parse_file (void)
+static void elna_langhook_parse_file(void)
 {
-  for (int i = 0; i < num_in_fnames; i++)
+    for (int i = 0; i < num_in_fnames; i++)
     {
-      elna_parse_file (in_fnames[i]);
+        elna_parse_file (in_fnames[i]);
     }
 }
 
@@ -161,39 +162,32 @@ elna_langhook_type_for_mode (enum machine_mode mode, int unsignedp)
   return NULL;
 }
 
-static tree
-elna_langhook_type_for_size (unsigned int bits ATTRIBUTE_UNUSED,
-			     int unsignedp ATTRIBUTE_UNUSED)
+static tree elna_langhook_type_for_size(unsigned int bits ATTRIBUTE_UNUSED,
+    int unsignedp ATTRIBUTE_UNUSED)
 {
-  gcc_unreachable ();
-  return NULL;
+    gcc_unreachable();
 }
 
 /* Record a builtin function.  We just ignore builtin functions.  */
 
-static tree
-elna_langhook_builtin_function (tree decl)
+static tree elna_langhook_builtin_function(tree decl)
 {
-  return decl;
+    return decl;
 }
 
-static bool
-elna_langhook_global_bindings_p (void)
+static bool elna_langhook_global_bindings_p(void)
 {
-  gcc_unreachable ();
-  return true;
+    gcc_unreachable();
 }
 
-static tree
-elna_langhook_pushdecl (tree decl ATTRIBUTE_UNUSED)
+static tree elna_langhook_pushdecl(tree decl ATTRIBUTE_UNUSED)
 {
-  gcc_unreachable ();
+    gcc_unreachable();
 }
 
-static tree
-elna_langhook_getdecls (void)
+static tree elna_langhook_getdecls(void)
 {
-  return NULL;
+    return NULL;
 }
 
 #undef LANG_HOOKS_NAME
diff --git a/include/elna/gcc/elna-generic.h b/include/elna/gcc/elna-generic.h
index 4254f02..c5ce340 100644
--- a/include/elna/gcc/elna-generic.h
+++ b/include/elna/gcc/elna-generic.h
@@ -30,6 +30,7 @@ namespace gcc
         void visit(source::integer_literal *literal) override;
         void visit(source::boolean_literal *literal) override;
         void visit(source::binary_expression *expression) override;
+        void visit(source::constant_definition *definition) override;
         void visit(source::declaration *declaration) override;
         void visit(source::variable_expression *expression) override;
         void visit(source::assign_statement *statement) override;
diff --git a/include/elna/gcc/elna-tree.h b/include/elna/gcc/elna-tree.h
new file mode 100644
index 0000000..883f2a2
--- /dev/null
+++ b/include/elna/gcc/elna-tree.h
@@ -0,0 +1,16 @@
+#pragma once
+
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "tree.h"
+
+enum elna_tree_index
+{
+  ELNA_TI_ENUMERAL_TYPE,
+  ELNA_TI_MAX
+};
+
+extern GTY(()) tree elna_global_trees[ELNA_TI_MAX];
+
+#define enumeral_node elna_global_trees[ELNA_TI_ENUMERAL_TYPE]