Prevention is usually thought of as relevant primarily for children and youths, not adults. For every drug, several potential targets exist in trajectories of drug use for prevention activities. These include 1) initiation; 2) continuation; 3) progression within a drug class to more extensive, intensive use patterns; 4) progression across drug classes; 5) progression to dependence; 6) regression to cessation; and 7) lapse and relapse. The preponderance of research on substance abuse prevention to date has focused on two targets: 1) attempts to reduce the likelihood of initiation (or at least delay onset) and prevalence of use among youths and 2) reducing the likelihood of relapse among adult drug abusers. In this chapter, we concentrate on primarily youth-focused prevention.