Collaborative systems such as Grids enable seamless resource sharing across multiple organisations, constituting the basic infrastructure for today’s data-intensive scientific collaborations. This talk presents security challenges when sharing data in Grids and introduces the usage control framework as a potential solution for such challenges. Usage control is a new authorisation paradigm that encompasses and extends several access control models. We introduce a process description language used as a policy specification language and show its suitability to model usage policies for Grids. We also describe the use of reputation systems for Grids and show how reputation can be combined with usage control. Our reputation model is based on utility computing, representing users’ feedback as utility functions that reflect the satisfaction a user perceives after consuming a service. It will be shown how the system can be used to rate users according to their compliance with resource usage policies.