Autonomic communication and computing is the new paradigm for dynamic service integration over a network. In an autonomic network, clients may have the right credentials to access a service but may not know it; equally, it is unrealistic to assume that service providers would publish their policies on the Web so that clients could do policy evaluation themselves. To solve this problem, the chapter proposes a novel interactive access control model: Servers should be able to interact with clients asking for missing credentials, whereas clients may decide to comply or not with the requested credentials. The process iterates until a final agreement is reached or denied. Further, the chapter shows how to model a trust negotiation protocol that allows two entities in a network to automatically negotiate requirements needed to access a service. A practical implementation of the access control model is given using X.509 and SAML standards.