The aim of this paper is to present a logical framework in which the concepts of trust and reputation can be formally characterized and their properties studied. We start from the definition of trust proposed by Castelfranchi & Falcone (C&F). We formalize this definition in a logic of time, action, beliefs and choices. Then, we provide a refinement of C&F’s definition by distinguishing two general types of trust: occurrent trust and dispositional trust. In the second part of the paper we present a definition of reputation that is structurally similar to the definition of trust but moves the basic concept of belief to a collective dimension of group belief.