Grids are potentially composed of several thousands of users from different institutions sharing their computing resources (or using resources provided by third parties). Controlling access to these resources is a difficult problem, as it depends on the policies of the organizations the users belong to and of the resource owners. Moreover, a simple authorization implementation, based on a direct user registration on the resources, is not applicable to a large scale environment. In this paper, we describe the solution to this problem developed in the framework of the European DataGrid M. Draoli, G. Mascari, R. Piccinelli, Project Presentation, DataGrid-11-NOT-0103-_1 and DataTAG http://www.datatag.org/ projects: the Virtual Organization Membership Service (VOMS) R. Alfieri, et al., Managing Dynamic User Communities in a Grid of Autonomous Resources, TUBT005, in: Proceedings of the CHEP 2003, 2003. VOMS allows a fine grained control of the use of the resources both to the users’ organizations and to the resource owners.