The paper begins by reviewing key conceptual and theoretical issues in the assessment of substance abusing offenders. In doing so, it draws on recent empirical research and theory in correctional treatment and substance abuse assessment and treatment to provide a critical commentary on deficiencies evident in correctional approaches to assessing offenders' alcohol and drug problems in common use today. The paper then highlights data on the nature, prevalence, and severity of alcohol and other drug problems among the offender population. It briefly examines the functional link between substance use and crime and discusses a number of treatment-relevant assessment dimensions. The discussion addresses some of the subgroups of substance abusing offenders in need of special attention. Issues in the validity of offender self-report information are discussed; and the roles of problem recognition, readiness for change, and motivation are addressed. Suggestions are offered for the use of assessment throughout the treatment process, and the concept of harm-reduction as an important consideration in assessing offenders is discussed. The paper concludes with a description of a proposed model for substance abuse assessment; it is based in the principle that, like other clinical populations, substance abusing offenders are heterogeneous with respect to their treatment needs and that assessment should guide the development of a unique treatment plan that connects the individual with treatment of appropriate intensity and duration. 13 figures, 5 notes, and 109 references