Psychoactive substance use has been part of people's lives for millennia (Austin 1978). About half of the world population uses at least one psychoactive substance, and although most do so without difficulties, for others problems arise that are related to the substance use (United Nations Office on Drugs and Crime 2005). Worldwide, drug and alcohol use disorders (excluding tobacco) are the sixth leading cause of disease burden in adults, whereas tobacco use and exposure to tobacco smoke are the leading preventable causes of death (World Health Organization 2003). Nationally (again excluding tobacco), 63% of American adults report that alcohol or drug addiction in themselves, family, or close friends has had an impact on their lives (Peter D. Hart Research Associates 2004). This chapter presents an overview of substance-related disorders, primarily focusing on those substances that are abused for their psychoactive effects, with additional material contained in the "Suggested Readings" listed at the end of the chapter.