diff --git a/de.churl.simple/bytecodeinterpreter.py b/de.churl.simple/bytecodeinterpreter.py
index d7b55e7..7e13102 100644
--- a/de.churl.simple/bytecodeinterpreter.py
+++ b/de.churl.simple/bytecodeinterpreter.py
@@ -1,8 +1,8 @@
-from simpleparser import parse
-from objspace import ObjectSpace
-import compile
 from disass import disassemble
 
+import compile
+from objspace import ObjectSpace
+
 
 class ByteCodeError(Exception):
     pass
@@ -46,7 +46,6 @@ class Interpreter(object):
                     w_condition = stack.pop()
                     if self.space.isfalse(w_condition):
                         pc += oparg
-                continue
             elif compile.hasarg(opcode):
                 oparg = ord(code[pc])
                 pc += 1
@@ -62,20 +61,24 @@ class Interpreter(object):
                     stack.append(obj)
                 elif opcode == compile.MAKE_OBJECT_CALL:
                     self.run(bytecode.subbytecodes[oparg], stack[-1])
+
+                # Project -----
                 elif opcode == compile.INT_LITERAL:
                     w_value = self.space.newint(oparg)
                     stack.append(w_value)
-                elif opcode == compile.BOOL_LITERAL:  # Project: Boolean
+                elif opcode == compile.BOOL_LITERAL:
                     w_value = self.space.newbool(oparg)  # oparg is 1 or 0
                     stack.append(w_value)
-                elif opcode == compile.STRING_LITERAL:  # Project: String
+                elif opcode == compile.STRING_LITERAL:
                     value = bytecode.symbols[oparg]
                     w_value = self.space.newstring(value)
                     stack.append(w_value)
-                elif opcode == compile.DOUBLE_LITERAL:  # Project: Double
+                elif opcode == compile.DOUBLE_LITERAL:
                     value = bytecode.symbols[oparg]
                     w_value = self.space.newdouble(value)
                     stack.append(w_value)
+                # -------------
+
                 elif opcode == compile.MAKE_FUNCTION:
                     bc = bytecode.subbytecodes[oparg]
                     w_method = self.space.definemethod(name=bc.name, code=bc, w_target=w_context)
@@ -85,7 +88,7 @@ class Interpreter(object):
                     w_method = self.space.getvalue(stack[-1], name)
                     stack.append(w_method)
                 elif opcode == compile.METHOD_CALL:
-                    arguments_w = [stack.pop() for n in range(oparg)]
+                    arguments_w = [stack.pop() for _ in range(oparg)]
                     arguments_w.reverse()
                     #
                     w_method = stack.pop()
@@ -94,7 +97,7 @@ class Interpreter(object):
                     stack.append(w_result)
                 elif opcode == compile.PRIMITIVE_METHOD_CALL:
                     nargs = self.space.get_number_of_arguments_of_primitive(oparg)
-                    arguments_w = [stack.pop() for n in range(nargs)]
+                    arguments_w = [stack.pop() for _ in range(nargs)]
                     arguments_w.reverse()
                     w_receiver = stack.pop()
                     w_result = self.space.call_primitive(oparg, w_receiver, arguments_w)
@@ -126,6 +129,11 @@ class Interpreter(object):
                     stack.append(w_context)
                 elif opcode == compile.DUP:
                     stack.append(stack[-1])
+
+                # Project
+                elif opcode == compile.GC:
+                    self.space.gc(w_context)
+
                 else:
                     raise ByteCodeError('Invalid bytecode')
         assert pc == len(code)
diff --git a/de.churl.simple/compile.py b/de.churl.simple/compile.py
index f6f4407..ba4c743 100644
--- a/de.churl.simple/compile.py
+++ b/de.churl.simple/compile.py
@@ -109,10 +109,10 @@ import simpleast
 
 # ---------- bytecodes ----------
 
-BOOL_LITERAL = 1  # 1 or 0 # Project: Boolean
+BOOL_LITERAL = 1  # 1 or 0
 INT_LITERAL = 2  # integer value
-STRING_LITERAL = 3  # Project: String
-DOUBLE_LITERAL = 17  # Project: Double
+STRING_LITERAL = 3
+DOUBLE_LITERAL = 17
 ASSIGNMENT = 4  # index of attrname
 METHOD_LOOKUP = 5  # index of method name
 METHOD_CALL = 6  # number of arguments
@@ -129,6 +129,7 @@ SET_LOCAL = 16  # index of attrname (optimization)
 IMPLICIT_SELF = 32  # (no argument)
 POP = 33  # (no argument)
 DUP = 34  # (no argument)
+GC = 35  # (no argument)
 
 opcode_names = [None] * 256
 for key, value in list(globals().items()):
@@ -191,9 +192,9 @@ def compile(ast, argumentnames=[], name=None):
 
 
 stack_effects = {
-    BOOL_LITERAL: 1,  # Project: Boolean
-    STRING_LITERAL: 1,  # Project: String
-    DOUBLE_LITERAL: 1,  # Project: Double
+    BOOL_LITERAL: 1,
+    STRING_LITERAL: 1,
+    DOUBLE_LITERAL: 1,
     INT_LITERAL: 1,
     ASSIGNMENT: -1,
     METHOD_LOOKUP: 1,
@@ -208,6 +209,7 @@ stack_effects = {
     IMPLICIT_SELF: 1,
     POP: -1,
     DUP: 1,
+    GC: 0,
 }
 
 
@@ -257,6 +259,7 @@ class Compiler(object):
             stackeffect = stack_effects[opcode]
         else:
             assert stackeffect != sys.maxsize
+
         self.stack_effect(stackeffect)
 
     def get_position(self):
@@ -289,18 +292,24 @@ class Compiler(object):
     def compile_IntLiteral(self, astnode, needsresult):
         self.emit(INT_LITERAL, astnode.value)
 
-    # Project: Boolean
+    # Project -----
     def compile_BooleanLiteral(self, astnode, needsresult):
         self.emit(BOOL_LITERAL, astnode.value)
 
-    # Project: String
     def compile_StringLiteral(self, astnode, needsresult):
         self.emit(STRING_LITERAL, self.lookup_symbol(astnode.value))  # save string value to symboltable
 
-    # Project: Double
     def compile_DoubleLiteral(self, astnode, needsresult):
         self.emit(DOUBLE_LITERAL, self.lookup_symbol(astnode.value))
 
+    def compile_GCStatement(self, astnode, needsresult):
+        self.emit(GC)
+
+        if needsresult:
+            self.emit(BOOL_LITERAL, True)
+
+    # -------------
+
     def compile_ImplicitSelf(self, astnode, needsresult):
         self.emit(IMPLICIT_SELF)
 
@@ -370,7 +379,7 @@ class Compiler(object):
         for statement in astnode.statements[:-1]:
             self.compile(statement, needsresult=False)
         laststatement = astnode.statements[-1]
-        self.compile(laststatement, needsresult)
+        self.compile(laststatement, needsresult)  # return last result
 
     def compile_FunctionDefinition(self, astnode, needsresult):
         bytecode = compile(astnode.block, astnode.arguments, astnode.name)
diff --git a/de.churl.simple/garbagecollection.py b/de.churl.simple/garbagecollection.py
new file mode 100644
index 0000000..010bd88
--- /dev/null
+++ b/de.churl.simple/garbagecollection.py
@@ -0,0 +1,18 @@
+def mark(w_context):
+    w_context.mark = True
+
+    if not hasattr(w_context, "slots"):  # skip primitive objects
+        return
+
+    for name, obj in w_context.slots.items():
+        if name != "__parent__":  # only descent
+            mark(obj)
+
+
+def sweep(objects):
+    objects[:] = filter(lambda obj: obj.mark, objects)  # inplace
+
+
+def clear_marks(objects):
+    for obj in objects:
+        obj.mark = False
diff --git a/de.churl.simple/objmodel.py b/de.churl.simple/objmodel.py
index ceb42fe..c2ed1bd 100644
--- a/de.churl.simple/objmodel.py
+++ b/de.churl.simple/objmodel.py
@@ -2,6 +2,9 @@ from c3computation import compute_C3_mro as c3
 
 
 class AbstractObject(object):
+    def __init__(self):
+        self.mark = False  # gc marker
+
     def call(self, w_receiver, args_w):
         return self
 
@@ -24,30 +27,9 @@ class AbstractObject(object):
         return c3(self)
 
 
-class PrimitiveObject(AbstractObject):
-    def __init__(self, value, trait, space=None):
-        self.value = value
-        self._trait = trait
-        self.space = space
-
-    def getparents(self):
-        if self.space is None:
-            return []  # for tests
-        trait = self.space.getbuiltin(self._trait)
-        assert trait is not None, 'O_o bogus state'
-        return [trait]
-
-    def hasslot(self, name):
-        return False
-
-    def __str__(self):
-        return str(self.value)
-
-    __repr__ = __str__
-
-
 class W_NormalObject(AbstractObject):
     def __init__(self, name=None, slots=None, parents=None, space=None):
+        super().__init__()
         self.space = space
         self.name = name
         if slots:
@@ -93,44 +75,59 @@ class W_NormalObject(AbstractObject):
             slots=self.slots.copy())
 
 
-class W_Integer(PrimitiveObject):
-    def __init__(self, value, space=None):
-        super().__init__(int(value), "inttrait", space)
+# Project -----
+class PrimitiveObject(AbstractObject):
+    def __init__(self, value, trait, space=None):
+        super().__init__()
+        self.value = value
+        self._trait = trait
+        self.space = space
 
-    def istrue(self):
-        return self.value != 0
+    def getparents(self):
+        if self.space is None:
+            return []  # for tests
+        trait = self.space.getbuiltin(self._trait)
+        assert trait is not None, 'O_o bogus state'
+        return [trait]
 
-
-# Project: Boolean
-class W_Boolean(PrimitiveObject):  # don't know if extending is good idea
-    def __init__(self, value, space=None):
-        super().__init__(int(value), "booltrait", space=space)
+    def hasslot(self, name):
+        return False
 
     def __str__(self):
-        return str(bool(self.value))
+        return str(self.value)
 
     __repr__ = __str__
 
     def istrue(self):
-        return self.value != 0
+        return bool(self.value)
+
+
+class W_Integer(PrimitiveObject):
+    def __init__(self, value, space=None):
+        super().__init__(int(value), "inttrait", space)
+
+
+class W_Boolean(PrimitiveObject):
+    def __init__(self, value, space=None):
+        super().__init__(int(value), "booltrait", space=space)
+
+    def __str__(self):
+        return str(bool(self.value))  # true instead of 1
+
+    __repr__ = __str__
 
 
-# Project: String
 class W_String(PrimitiveObject):
     def __init__(self, value, space=None):
         super().__init__(str(value), "strtrait", space)
 
-    def istrue(self):
-        return self.value != ""
 
-
-# Project: Double
 class W_Double(PrimitiveObject):
     def __init__(self, value, space=None):
         super().__init__(float(value), "doubletrait", space)
 
-    def istrue(self):
-        return self.value != 0.
+
+# -------------
 
 
 class W_Method(W_NormalObject):
diff --git a/de.churl.simple/objspace.py b/de.churl.simple/objspace.py
index cee4110..ccbfd04 100644
--- a/de.churl.simple/objspace.py
+++ b/de.churl.simple/objspace.py
@@ -1,14 +1,15 @@
+import primitives
+from garbagecollection import mark, sweep, clear_marks
 from objmodel import W_Integer, W_Boolean, W_String, W_Double
 from objmodel import W_Method
 from objmodel import W_NormalObject
 
-import primitives
-
 
 class ObjectSpace(object):
 
     def __init__(self, interpreter):
         self.interpreter = interpreter
+        self.objects = []
 
     def setup_builtins(self, builtincode=None):
         if builtincode is None:
@@ -20,6 +21,7 @@ class ObjectSpace(object):
         ast = parse(builtincode)
 
         self.interpreter.eval(ast, w_builtins)
+        self.objects.clear()  # remove all builtins, lobby from list
 
     def _load_default_builtins(self):
         import os
@@ -40,32 +42,43 @@ class ObjectSpace(object):
             slots = {}
         else:
             slots = {'__parent__': self.getbuiltins()}
-        return W_NormalObject(name=name, slots=slots)
+
+        return W_NormalObject(name=name, slots=slots)  # lobby isn't collected
 
     def newobject(self, name, slots, parentnames):
-        return W_NormalObject(space=self, name=name,
-                              slots=slots, parents=parentnames)
+        self.objects.append(W_NormalObject(space=self, name=name,
+                                           slots=slots, parents=parentnames))
+        return self.objects[-1]
 
+    # Project -----
     def newint(self, value):
-        return W_Integer(value, space=self)
+        self.objects.append(W_Integer(value, space=self))
+        return self.objects[-1]
 
-    # Project: Boolean
     def newbool(self, value):
-        return W_Boolean(value, space=self)
+        self.objects.append(W_Boolean(value, space=self))
+        return self.objects[-1]
 
-    # Project: String
     def newstring(self, value):
-        return W_String(value, space=self)
+        self.objects.append(W_String(value, space=self))
+        return self.objects[-1]
 
-    # Project: Double
     def newdouble(self, value):
-        return W_Double(value, space=self)
+        self.objects.append(W_Double(value, space=self))
+        return self.objects[-1]
+
+    def gc(self, w_context):
+        clear_marks(self.objects)
+        mark(w_context)
+        sweep(self.objects)
+
+    # -------------
 
     def definemethod(self, name, code, w_target):
-        w_meth = W_Method(code, name=name,
-                          slots={'__parent__': w_target},
-                          space=self)
-        return w_meth
+        self.objects.append(W_Method(code, name=name,
+                                     slots={'__parent__': w_target},
+                                     space=self))
+        return self.objects[-1]
 
     def execute(self, code, w_context):
         return self.interpreter.run(code, w_context)
diff --git a/de.churl.simple/simpleast.py b/de.churl.simple/simpleast.py
index a53f840..f84db06 100644
--- a/de.churl.simple/simpleast.py
+++ b/de.churl.simple/simpleast.py
@@ -93,7 +93,7 @@ class IntLiteral(Expression):
         self.value = int(value)
 
 
-# Project: Boolean
+# Project -----
 class BooleanLiteral(Expression):
     """ A boolean literal (like "false") """
 
@@ -103,7 +103,6 @@ class BooleanLiteral(Expression):
         self.value = value == "true"
 
 
-# Project: String
 class StringLiteral(Expression):
     """ A string literal (like "hello world") """
 
@@ -113,7 +112,6 @@ class StringLiteral(Expression):
         self.value = str(value)
 
 
-# Project: Double
 class DoubleLiteral(Expression):
     """ A double literal (like "1.0", ".0", "1.", "+1.0") """
 
@@ -123,6 +121,9 @@ class DoubleLiteral(Expression):
         self.value = float(value)
 
 
+# -------------
+
+
 class MethodCall(Expression):
     """ A call to a method with name 'methodname' on 'receiver' with
     'arguments' (which is a list of expression ASTs).
@@ -172,6 +173,11 @@ class Statement(AstNode):
     """ Base class of all statement nodes. """
 
 
+# Project
+class GCStatement(Statement):
+    """ Triggers Garbage Collection """
+
+
 class Assignment(Statement):
     """ An assignement: lvalue attrname = expression.
 
diff --git a/de.churl.simple/simplelexer.py b/de.churl.simple/simplelexer.py
index 04b9ddf..3bfd504 100644
--- a/de.churl.simple/simplelexer.py
+++ b/de.churl.simple/simplelexer.py
@@ -158,7 +158,7 @@ OpenBracket = r'[\[\(\{]'
 CloseBracket = r'[\]\)\}]'
 
 # ____________________________________________________________
-# Project: Boolean, String, Double
+# Project
 
 Boolean = r"true|false"
 String = group(make_single_string(r"\'"), make_single_string(r'\"'))
@@ -166,11 +166,8 @@ String = group(make_single_string(r"\'"), make_single_string(r'\"'))
 _sign = r"([+-])?"
 _int = r"(([1-9][0-9]*)|0)"
 _dec = r"(([0-9]*[1-9])|0)"
-Double = group(_sign + group(_int, r"") + r"\." + _dec,
-               _sign + _int + r"\." + group(_dec, r""))
-
-# ____________________________________________________________
-# Project: Sugar
+Double = group(_sign + group(_int, r"") + r"\." + _dec,  # 0.1 / .1
+               _sign + _int + r"\." + group(_dec, r""))  # 1.0 / 1.
 
 Plus = r'\+'
 Minus = r'-'
@@ -179,6 +176,8 @@ Divide = r'/'
 Increment = r'\+\+'
 Modulo = r'%'
 
+GC = r'gc'
+
 # ____________________________________________________________
 # Keywords
 
@@ -189,10 +188,11 @@ Def = r'def'
 Object = r'object'
 
 tokens = ["If", "Else", "While", "Def", "Object", "Ignore",
-          "String", "Boolean", "Double",  # Project: Boolean, String, Double
+          "String", "Boolean", "Double",
           "Number",  # after Double
+          "GC",
           "NewlineAndWhitespace", "OpenBracket", "CloseBracket", "Comma", "Assign", "Colon",
-          "Increment", "Plus", "Minus", "Multiply", "Divide", "Modulo",  # Project: Sugar
+          "Increment", "Plus", "Minus", "Multiply", "Divide", "Modulo",
           "Name", "PrimitiveName"]
 
 
diff --git a/de.churl.simple/simpleparser.py b/de.churl.simple/simpleparser.py
index eb99719..025ee03 100644
--- a/de.churl.simple/simpleparser.py
+++ b/de.churl.simple/simpleparser.py
@@ -6,10 +6,11 @@ from simplelexer import lex
 import simpleast
 
 pg = ParserGenerator(["If", "Else", "While", "Def", "Object", "Number",
-                      "String", "Boolean", "Double",  # Project: Boolean, String, Double
+                      "String", "Boolean", "Double",
+                      "GC",
                       "Name", "Indent", "Dedent", "Newline", "OpenBracket",
                       "CloseBracket", "Comma", "Assign", "Colon",
-                      "Increment", "Plus", "Minus", "Multiply", "Divide", "Modulo",  # Project: Sugar
+                      "Increment", "Plus", "Minus", "Multiply", "Divide", "Modulo",
                       "PrimitiveName", "EOF"],
                      # Operator precedence for ambiguous rules, ascending
                      precedence=[("left", ["Plus", "Minus"]),
@@ -63,6 +64,12 @@ def newlines(n):
     return None
 
 
+@pg.production("statement : GC newlines")
+@pg.production("statement : GC")
+def gcstatement(stmt):
+    return simpleast.GCStatement()
+
+
 @pg.production("statement : simplestatement")
 @pg.production("statement : ifstatement")
 @pg.production("statement : whilestatement")
@@ -198,19 +205,16 @@ def number_expression(stmt):
     return simpleast.IntLiteral(stmt[0].value)
 
 
-# Project: Boolean
 @pg.production("basic_expression : Boolean")
 def boolean_expression(stmt):
     return simpleast.BooleanLiteral(stmt[0].value)
 
 
-# Project: String
 @pg.production("basic_expression : String")
 def string_expression(stmt):
     return simpleast.StringLiteral(stmt[0].value[1:-1])  # cut off delimiters
 
 
-# Project: Double
 @pg.production("basic_expression : Double")
 def double_expression(stmt):
     return simpleast.DoubleLiteral(stmt[0].value)
diff --git a/mytests/test_gc.py b/mytests/test_gc.py
index bf97dfb..48fe235 100644
--- a/mytests/test_gc.py
+++ b/mytests/test_gc.py
@@ -4,33 +4,36 @@ from interpreter import Interpreter
 
 def test_reassignment_gc():
     ast = parse("""
+true
 x = 2
 y = 3
 """)
     interpreter = Interpreter()
     w_model = interpreter.make_module()
     interpreter.eval(ast, w_model)
+    interpreter.space.gc(w_model)
 
     x = w_model.getvalue("x")
     y = w_model.getvalue("y")
-    assert x in interpreter.space.realm  # Wo alle Objekte leben
-    assert y in interpreter.space.realm
+    assert x in interpreter.space.objects
+    assert y in interpreter.space.objects
 
     ast = parse("""
 x = y
 """)
     interpreter.eval(ast, w_model)
-    interpreter.space.gc()
-    assert x not in interpreter.space.realm
-    assert y in interpreter.space.realm
+    interpreter.space.gc(w_model)
+    assert x not in interpreter.space.objects
+    assert y in interpreter.space.objects
 
     ast = parse("""
 x = 0
+y = 0
 """)
     interpreter.eval(ast, w_model)
-    interpreter.space.gc()
-    assert x not in interpreter.space.realm
-    assert y not in interpreter.space.realm
+    interpreter.space.gc(w_model)
+    assert x not in interpreter.space.objects
+    assert y not in interpreter.space.objects
 
 
 def test_chain_gc():
@@ -42,40 +45,25 @@ z = y
     interpreter = Interpreter()
     w_model = interpreter.make_module()
     interpreter.eval(ast, w_model)
+    interpreter.space.gc(w_model)
 
     x = w_model.getvalue("x")
-    assert x in interpreter.space.realm
+    assert x in interpreter.space.objects
 
     ast = parse("""
 x = 0
 """)
     interpreter.eval(ast, w_model)
-    interpreter.space.gc()
-    assert x in interpreter.space.realm
+    interpreter.space.gc(w_model)
+    assert x in interpreter.space.objects
 
     ast = parse("""
 y = x
 z = y
 """)
     interpreter.eval(ast, w_model)
-    interpreter.space.gc()
-    assert x not in interpreter.space.realm
-
-
-def test_while_gc():
-    ast = parse("""
-x = 10
-while x:
-    x = x $int_add(-1)
-""")
-    interpreter = Interpreter()
-    w_model = interpreter.make_module()
-    interpreter.eval(ast, w_model)
-
-    count = len(interpreter.space.realm)
-    interpreter.space.gc()
-
-    assert count - len(interpreter.space.realm) == 10
+    interpreter.space.gc(w_model)
+    assert x not in interpreter.space.objects
 
 
 def test_object_gc():
@@ -88,19 +76,76 @@ object x:
     interpreter = Interpreter()
     w_model = interpreter.make_module()
     interpreter.eval(ast, w_model)
+    interpreter.space.gc(w_model)
 
     a = w_model.getvalue("x").getvalue("a")
     b = w_model.getvalue("x").getvalue("b")
     c = w_model.getvalue("x").getvalue("c")
-    assert a in interpreter.space.realm
-    assert b in interpreter.space.realm
-    assert c in interpreter.space.realm
+    x = w_model.getvalue("x")
+    assert a in interpreter.space.objects
+    assert b in interpreter.space.objects
+    assert c in interpreter.space.objects
+    assert x in interpreter.space.objects
 
     ast = parse("""
 x = 0
 """)
     interpreter.eval(ast, w_model)
-    interpreter.space.gc()
-    assert a not in interpreter.space.realm
-    assert b not in interpreter.space.realm
-    assert c not in interpreter.space.realm
+    interpreter.space.gc(w_model)
+    assert a not in interpreter.space.objects
+    assert b not in interpreter.space.objects
+    assert c not in interpreter.space.objects
+    assert x not in interpreter.space.objects
+
+
+def test_method_gc():
+    ast = parse("""
+def meth:
+    1
+""")
+    interpreter = Interpreter()
+    w_model = interpreter.make_module()
+    interpreter.eval(ast, w_model)
+    interpreter.space.gc(w_model)
+
+    meth = w_model.getvalue("meth")
+    assert meth in interpreter.space.objects
+
+    ast = parse("""
+def meth:
+    2
+""")
+    interpreter.eval(ast, w_model)
+
+    assert meth in interpreter.space.objects
+
+    interpreter.space.gc(w_model)
+
+    assert meth not in interpreter.space.objects
+
+
+def test_simple_call_gc():
+    ast = parse("""
+x = 1
+gc
+""")
+    interpreter = Interpreter()
+    w_model = interpreter.make_module()
+    interpreter.eval(ast, w_model)
+
+    x = w_model.getvalue("x")
+    assert x in interpreter.space.objects
+
+    ast = parse("""
+x = 2
+""")
+    interpreter.eval(ast, w_model)
+
+    assert x in interpreter.space.objects
+
+    ast = parse("""
+gc
+""")
+    interpreter.eval(ast, w_model)
+
+    assert x not in interpreter.space.objects