This chapter examines the main theories of concepts that are currently on offer in the cognitive science literature. So much of the cognitive science literature is devoted to the nature of concepts, conceptual structures, and conceptual content. Once a psychologist has made up his mind about concepts, much of the rest of what he says about cognition is a forced option. However, proposing constraints on theories of concepts is different from proposing an empirically adequate theory that satisfies the constraints. This chapter first considers the “dual code theory” of mental representations, with particular emphasis on its assumption that some concepts (or at least some tokens of concepts) consist of mental words in one's native language while other concepts consist of mental pictures. It then discusses some of the reasons why concepts can't be images before turning to other options for representing concepts. It also looks at concepts as stereotypes and as inferential roles, as well as whether concepts are locations in associative or “semantic” networks. Finally, it argues that all the available accounts of conceptual content seem to be not viable.