Offender classification schemes vary from State to State and also among correctional facilities within the same jurisdiction. The assessment of alcohol and drug-involved offenders should involve multiple levels. The role of assessment is to determine what approach is best for each offender. Several assessment approaches are identified: (1) treatment needs assessments to identify what type of program intervention is appropriate; (2) readiness for treatment assessments to understand the extent to which clients are motivated for treatment and whether they will benefit; (3) comprehensive treatment planning assessments once a client is placed in a program to determine how intensive treatment should be; (4) periodic treatment progress assessments to determine how clients are responding to treatment and whether intervention changes should be considered; and (5) treatment outcome assessments to evaluate the extent of behavioral change, success, and failure. Clinical instruments available for conducting assessments are noted.