Pervasive Grids emerge as a new paradigm for providing nomadic users with ubiquitous access to digital information and computing resources. However, pervasive grids arise a number of crucial issues related to privacy and security, especially authentication and access control, which constitute the security front-end. In this paper, we propose a trust based model of authentication and access control that allows nomadic users to roam from site to site and to gain access to surrounding/remote resources wrt her status in her home site and to the local policy of the site where she is standing. This model is supported by a software architecture called Chameleon. The Chameleon permits users to access grid resources and to implement adhoc interactions with the local grid site.