This research compares the degree, mechanisms, and processes of control over nonmedical opiate use across three sample groups of regular and current users: controlled users (N equals 61), marginal users (N equals 7), and compulsive users not in treatment (N equals 30). These groups represent a spectrum of heroin-using styles ranging from moderate and occasional, i.e., "controlled," use at one extreme to very frequent and physiologically addictive use at the other extreme. The groups are defined according to quantitative criteria which encompass amount of opiate use (occasions of use), intervals between using occasions, how recently subjects used opiates, and the stability of the using pattern over time. Data are gathered by means of in-depth interviews at one-year intervals. Data for some subjects are verified by means of interviews with one or more "friends." Degree of control, the primary dependent variable, is measured by frequency of use, using practices, stability over time, and other factors. Analysis will emphasize the relation between setting variables (the characteristics of the physical and social environment in which use takes place) and control (especially how one using style evolves into another). Findings should assist current treatment and prevention efforts and suggest new strategies.