Thematic roles provide one way of relating situations to their participants. Thematic roles have been widely invoked both within lexical semantics and in the syntax-semantics interface, in accounts of a wide range of phenomena, most notably the mapping between semantic and syntactic arguments (argument realization). This article addresses two sets of issues. The first concerns the nature of thematic roles in semantic theories: what are thematic roles, are they specific to individual predicates or more general, how do they figure in semantic representations, and what interactions do they have with event and object individuation and with the semantics of plurality and aspect? The second concerns properties of systems of thematic roles: what is the inventory of thematic roles, and what relationships, such as an ordering of roles in a thematic hierarchy, or the consistency of roles across semantic domains posited by the thematic relations hypothesis, exist among roles? Various applications of thematic roles will be noted throughout these two sections, in some cases briefly mentioning alternative accounts that do not rely on them. The conclusion notes some skepticism about the necessity for thematic roles in linguistic theory.