Agent: A generic term representing any entities (e.g., peer in Peer-to-Peer networks, user in various Web 2.0 applications) that interact with each other Interaction or transaction: The process where the trustor accepts and consumes services or products that are provided by the trustee Trustee or target agent: The agent who will be evaluated Trustor: The agent who wants to evaluate its trust in another agent Overview Many Internet-scale social applications have evolved recently, fueled by and fueling applications open to all, e.g., various Web 2.0 applications, where end users play active role in the system, and thus, their activities in turn significantly impact the system’s behavior and other users’ experience with the same. For instance, different from traditional static websites, Web 2.0 applications allow users to not only retrieve information but also contribute contents and share information with others (e.g., Wikipedia, Facebook, Twitter,….