Other substances have been judged harmful and hence have been placed under strict control. This is the case for narcotic drugs and psychotropic substances controlled under the international drug control conventions. Irrespective of the level of control, regulation and societal approval or disapproval, one factor common to all psychoactive substances, referred to in this chapter as “drugs” for the sake of brevity, is their propensity to lead to drug use disorders following use, whether frequent or occasional. Drug use disorders are associated with significant levels of disease (morbidity) and disability, constitute a burden on national resources, and cause immeasurable human suffering.