Addiction is compulsive need for and use of a habit-forming substance. Addiction is accepted as a mental illness in the diagnostic nomenclature and results in substantial health, social and economic problems. In the diagnostic nomenclature, addiction was originally included in the personality disorders along with other behaviours considered deviant. But it is now considered a clinical syndrome. Addiction is multifactorially determined, with substantial genetic influence. The development of addictions is also influenced by environmental factors, and interplay between the two. In the clinical context, addiction puts problem substance use on the agenda, and helps focus on the difficulties associated with drug use. But the concept of addiction is also used to distance the user from addicts, and in this way, may be counter-therapeutic. The addiction concept has also had a substantial influence on policy. The almost universal prohibition against drugs such as opiates, cocaine, cannabis and amphetamine has much support. But unfortunately, it has not been able to hinder the development of substance problems. Optimism is fostered by the development of respectful ways of thinking about people with addictions, in particular, from advocates of motivational interviewing.