An obvious preliminary to the discussion of models of “drug addiction” is to consider what we mean by that term. Over the past several decades, a number of definitions of drug addiction have been used. Although some of the historically earliest definitions of addiction or drug dependence relied in part on the consequences of protracted drug exposure as defining characteristics, contemporary definitions generally recognize that use of a psychoactive drug itself is the central element in the process of addiction (see review in Clarke et al., 1989 ). For example, the recent report of the US Surgeon General on Nicotine Addiction used three primary criteria in defining addiction (US DHHS, 1988 ):