“In practice, psychiatrists often examine a patient who has nothing but delusions. Such patients have no marked depressive symptomatology, although they may be unhappy. They are not euphoric though they may be grandiose as a natural response to the delusion. They are neither blunted nor inappropriate in affect. This apparently simplistic, though largely accurate, description of paranoid disorders by George Winokur is in sharp contrast to the often confusing historical evolution of the concept of delusional (paranoid) disorders. This chapter covers the history and evolution of the concept, comparative nosologic viewpoints, diagnostic criteria for delusional disorder, epidemiological study of delusional disorders, clinical features of delusional disorders, differential diagnosis of delusional disorders, psychological tests, management, and cognitive therapy.