Abstract: A multi-domain resource access control mechanism uses a single access control system to manage access by users to resources that belong to multiple domains. A server is associated with each domain in a set of domains. Access to resources in the domains is governed by an access control system. A first server for a first domain transmits a data token to a client seeking access to a resource in a second domain. The client transmits the data token to a second server in the other domain. The second server uses the data token to verify that the user is authentic, that is, authorized to access resources protected by the access control system. Once determining that the user is authorized to access resources, access control cookies are transmitted to client. When the client requests access to a resource in the second domain, and the request did not include access control cookies for the second domain, data is transmitted to the browser causing it to generate another request to the first server.