GUMS, which stands for Grid User Management System, is a grid identity mapping service. Identity mapping is necessary when a grid site's resources do not use grid credentials but instead use a different mechanism to identify users, such as UNIX accounts or Kerberos principals. In these cases, the grid credential for each incoming computing job must be associated with an appropriate site credential. The GUMS server performs this mapping and communicates it to the gatekeepers. GUMS was developed at Brookhaven National Laboratory in the United States, and is part of the authorization infrastructure for the Open Science Grid.