Professionals in criminal justice, substance abuse, and mental health have become increasingly aware of the interaction between their fields and realize that dually disordered prisoners constitute the bulk of the "new prisoner" population. Clinical experience and research show that psychiatric patients who do not abuse drugs or alcohol are no more likely to commit crimes than the general public and that alcohol is responsible for more criminal behavior than any other drug. Underaged drinking and the use of such drugs as marijuana, cocaine, heroin, LSD, and PCP often lead to criminal activities. Further, psychiatric disorders and their symptoms frequently lead to drug or alcohol abuse which may, in turn, lead to crime. Psychiatric patients who abuse drugs or alcohol have an increased incidence of severe, sometimes violent, psychotic episodes. In addition, the abuse of drugs or alcohol is highly correlated with vehicular homicide and spouse/child abuse. The importance of additional information on the nature of psychiatric symptoms and disorders is stressed, particularly in the context of improved criminal justice system decisionmaking. Classification guidelines relevant to criminality, drug and alcohol use, and mental illness are offered.