Trust is a crucial concept in order to address scalability in managing security tasks such as authentication, authorization and access control. In this paper we discuss the concept of trust by defining a general theoretical model to describe basic trust relationships in heterogeneous environments composed of several Administrative Domains (ADs), i.e. autonomous domains for both security and administrative issues. As a result, we introduce the design of a Trust Management System (TMS) that we plan to deploy into a middleware architecture in order to provide a framework for implementing trust relationships in Virtual Enterprises (VEs).