This chapter reviews basic findings of the psychology of concepts that are the basis for two very different strands of research that investigate concepts, one focusing on formal aspects of categories and studying artificial category learning, the other focusing on the content of concepts and how learning and judgment interact with prior knowledge. Then it discusses formal models of category learning, including D.L. Medin and M.M. Schaffer's context model (CM). The formal models of category learning are turning to mixtures of different processes, with the hope that they can predict when one form of learning (rule testing, prototype extraction, exemplar learning) is preferred. Transitioning toward the second strand, the chapter discusses how higher-level knowledge influences the category learning task, suggesting that a broader approach may be required. Finally, the chapter moves completely to the second strand and reviews work on conceptual development, essentialism, and knowledge effects.