Those suffering from delusional disorders may seem harmless or eccentric until they commit a crime. It is for this reason that knowledge about delusional disorders is of great importance. It is also important that those qualified to make such diagnoses in order to determine the criminal liability of the accused, should correctly diagnose the specific delusional disorder. However, criminologists need enough information about the disorder to assist clinical staff in the preparation of court reports should an accused commit a crime “under the influence” of a delusion. In this paper, actual case studies of delusional disorders, within the first five main subtypes identified under the APA/DSM-IV-TR are applied to crimes that are likely to be committed when acting under these delusions, such as erotomanic and stalking, grandiose and defamation and forgery, jealous and domestic violence, persecutory and violence, and somatic and violent crime. In some instances delusions elicit behavior that may be criminal in nature. The concept “delusional disorder” derives from the Greek work paranous, meaning paranoia. According to the APA/DSM-IV-TR, the diagnosis of delusional disorders should meet five criteria (A through E). In addition, there are seven types of delusional disorders: erotomanic, grandiose, jealous, persecutory, somatic, mixed, and unspecified. Other delusions identified include: delusions of control, delusions of reference, and delusions of self-accusation. Delusional disorders generally affect individuals in middle or late adulthood, ranging from 18 to 90 years of age. When there are instances where delusions elicit behavior that is harmful to other and is considered criminal in nature, the study of delusions becomes important to criminology. References