This issue of Alcohol Research & Health examines addiction to multiple substances--that is, combined dependence on alcohol and other drugs (AODs), including marijuana, cocaine, and opioids. It seems fitting, then, to begin the issue with a look at what constitutes "addiction." The Oxford English Dictionary (pp. 24-25) traces the term addiction to Roman law, under which addiction was a "formal giving over by sentence of court; hence, a dedication of person to a master." This notion of relinquishment of control by the addicted person is the central feature of many lay and professional definitions of the term. The study of addictive behavior crosses several disciplines, including, among others, behavioral neuroscience, epidemiology, genetics, molecular biology, pharmacology, psychology, psychiatry, and sociology. Articles in this issue examine aspects of AOD use disorders from the perspective of some of these varied disciplines.