In the chapter on comorbid substance abuse and personality disorders, the authors discuss the different conceptual models of addiction and their implications for treatment and outcome. They look at questions of causality. When does personality disorder cause addiction? Does addiction cause personality disorder? The authors who address comorbid schizophrenia and substance abuse emphasize the special problems in treatment because of the apathy, cognitive impairment, and limited interpersonal skills of patients with schizophrenia. Substances of abuse may appeal to patients with schizophrenia not only because they stimulate pleasure centers in the brain but also because they improve brain functioning temporarily. The three psychologists who are responsible for the chapter on comorbid nicotine dependence and psychiatric and other substance use disorders focus on the role of nicotine in major depressive disorder, schizophrenia, and alcohol abuse. The chapter on traumatic brain injury examines its connections with alcohol abuse in particular, both from the point of view of causation and the effects of alcohol on glutamate and catecholamines in the brain. A group from South Carolina summarizes the research on the overlap of adult ADHD and alcoholism, substance abuse, and personality disorders. The chapter on eating disorders points up a kind of substance abuse that is unique to these disorders, namely, the use of diuretics, laxatives, and emetics. The chapter on comorbid trauma exposure, posttraumatic stress disorder (PTSD), and addictions discusses the current explanatory models. The author also reviews the evidence for neurobiological elements that are common to PTSD and substance dependence.