Although delusion remains one of the basic problems in psychopathology, attempts to understand its pathogenesis have been dominated by unsubstantiated speculation. Previous psychodynamic formulations have recently given way to increasing interest in measurement, and testing of models derived from cognitive psychology. However, the formation, elaboration, and persistence of delusional beliefs may be an expression of the convergence of numerous causal influences, each exerting an effect at a different stage in the evolution of the belief. This review takes a structured overview of the literature, placing the numerous part theories and scant experimental findings within a general model of delusional development. It argues for a return to systematic research on symptoms rather than complex diagnostic formulations to facilitate better understanding of the development of delusional disorders and stimulate further interest in therapeutic intervention.