Despite the wide adoption by the scientific community, grid technologies have not been given the appropriate attention by enterprises. This is merely due to the lack of enough studying and defining security requirements of grid computing systems. More specifically, access control in grid systems has been addressed with the same models for collaborative systems based on distributed computing across multiple administrative domains. However, existing solutions are not based on a foundation for a holistic approach in grid access control. This paper aims to provide an adequate approach in this direction. Additionally, a comparative review of current access control models is provided in the context of our proposed four-layer conceptual grid categorization.