Delusional thinking in psychotics is discussed in terms of definition, classification, and relevance to diagnosis. Theoretical explanations of delusions are then reviewed. While many explanations and theories on delusional thinking exist, some merely restate in different terms the same relationships, and most do not enjoy empirical support. It is concluded that existing theories, can be usefully organized into two main themes: motivational and defect. The motivational theme assumes that the individual develops the delusion either to explain unusual perceptual experiences or to reduce uncomfortable psychic states (e.g., unfullfilled need). The defect theme argues that delusional thinking results from some fundamental cognitive-attentional deficit. Future empirical work ought to pursue specific models which address some important distinctions among delusion types.