 Concepts are the building blocks of thoughts. Consequently, they are crucial to such psychological processes as categorization, inference, memory, learning, and decision-making. This much is relatively uncontroversial. But the nature of concepts—the kind of things concepts are—and the constraints that govern a theory of concepts have been the subject of much debate (Margolis &amp; Laurence 1999, Margolis &amp; Laurence 2015). This is due, at least in part, to the fact that disputes about concepts often reflect deeply opposing approaches to the study of the mind, to language, and even to philosophy itself. In this entry, we provide an overview of theories of concepts, and outline some of the disputes that have shaped debates surrounding the nature of concepts. The entry is organized around five significant issues that are focal points for many theories of concepts. Not every theory of concepts takes a stand on each of the five, but viewed collectively these issues show why the theory of concepts has been such a rich and lively topic in recent years. The five issues are: (1) the ontology of concepts, (2) the structure of concepts, (3) empiricism and nativism about concepts, (4) concepts and natural language, and (5) concepts and conceptual analysis.