In this dissertation, previous work on access control for both collaborative and non-collaborative systems is surveyed. New access control requirements for general collaborative environments are identified, and it is shown that existing models do not completely meet these requirements. A new access control model is developed for meeting the requirements. In particular, a set of collaboration rights are identified based on a general collaboration model; exception-based, multiple inheritance mechanisms are used to support both flexible and high-level access specification; and dynamic, multiple ownership rules are developed to support flexible access administration. The model can emulate a variety of existing systems and meets the new access requirements. It has been implemented in a generic, extensible collaborative system, which relieves individual applications from implementing the model.