Delusional disorders (DDs) are clinically rare syndromes characterized by false beliefs that are held with firm conviction despite counterevidence. The neuropsychology of DDs is poorly understood. Two partially opposing models—a cognitive bias model and a cognitive deficit model—have received mixed empiric support, partly because most research has been carried out in patients with paranoid schizophrenia, with which the nosologic association of DDs is unknown. Based on these models, we review empiric findings concerning the neuropsychology of DDs (narrowly defined). We conclude that DDs can best be seen as extreme variations of cognitive mechanisms involved in rapid threat detection and defensive harm avoidance. From this viewpoint, the two models seem to be complementary in explanatory power rather than contradictory. Future research may help to clarify the question of gene-environment interaction involvement in the formation of delusional beliefs.