Earlier work in this field is reviewed and present concepts and terminologies are examined in detail. A revised way of dealing with ideas implicit in the terms "drug abuse" or "drug misuse" is proposed; the term "neuroadaptive state" is suggested as an alternative to "physical dependence"; a profile is given of the elements that constitute a "drug dependence syndrome"; and the need to differentiate conceptually between "dependence" and "drug related disability" is stressed. A model of dependence is outlined in which dependence is considered as a psycho-physiological-social syndrome determined and kept going by a complex system of reinforcements. The association between substance use and mental illness is discussed with emphasis on possible two-way relationships. The need for population studies and in particular for longitudinal studies is emphasized. The bearing of all the fore-going considerations on work towards the revision of relevant classification systems is considered, and, in the final section, several recommendations on nomenclature are brought together and suggestions are made for research that might lead to refinement of classification and diagnostic systems.