In federated and pervasive networks, trust management has become a cornerstone for information security and privacy. Although people have recognized the importance of privacy and security for their personal information, they remain uncertain when they have to define and enforce their own access control rules or have to handle indirect information. Indirect information and subjective judgment are the major sources of uncertainty in federated trust management. This paper introduces fuzzy logic into the definition and evaluation of trust, and then provides a formal representation of fuzzy rules. It also offers a set of derivation rules for analyzing and reasoning among fuzzy rules in order to enforce these rules with a certain level of uncertainty. Application of this model to a healthcare environment with pervasive computing devices across trust domains provides a new method to handle uncertainty in trust management for federated and pervasive networks.