The present paper examines the limitations that are placed on our understanding of addiction when research is restricted to studying opiate users who are seeking treatment. Though users who do seek treatment do not seem to differ in terms of their history or magnitude of drug use from those not seeking treatment, those in treatment are likely to have more serious addiction-related problems. Addicts who seek different types of treatment (e.g., methadone maintenance vs therapeutic community) appear to differ in terms of social and psychological characteristics. However, specific characteristics do not appear to be matched with specific treatments. Thus the addicts seen in treatment in general or in any specific treatment represent a discrete subpopulation of the entire population of opiate users. As such, any generalizations that we make about addiction based on these addicts must be limited.