Sikkel, Klaas2017-04-152017-04-151997978-94-015-7372-6Requirements for access control in CSCW systems have often been stated, but groupware in use today does not meet most of these requirements. There are practical reasons for this, but one of the problems is the inherent complexity of sophisticated access control models. We propose a general authorization model that emphasizes conceptual simplicity and show that several issues—in particular negative access rights and delegation of rights—can be solved elegantly in this model.enText