Interpersonal trust is a mental construct with implications for social functioning and economic behavior. We review contemporary theories of trust from behavioral economics and social psychology. Neoclassical economic theory considers trust in strangers to be irrational, but observed behavior reveals widespread trust and trustworthiness. Theories of social preferences and adherence to social norms have been proposed to rationalize trust. Psychological approaches investigate trusting behavior in terms of an underlying disposition, intergroup processes, and cognitive expectations. The breadth of these approaches illustrates the multi-faceted nature of trusting behavior. The determinants of trust are related to the relevant characteristics of the individual, the situation, and their interaction.