The article describes a new standardized assessment battery of offender responsivity and identifies and discusses responsivity-related factors in terms of their potential impact on treatment outcome. The article places the construct of treatment responsivity in a context that underscores the importance of allocating offenders to programs in the most effective manner and of identifying factors that might mediate the effectiveness of treatment services. The article concludes that best practice with regard to responsivity starts with good assessment. Knowing an offender's motivation level, cognitive ability, personality traits, and maturity is essential to good case planning. A good case plan also takes into account factors related to treatment settings, treatment program options, and staff characteristics. Finally, understanding staff skills and interests should also become part of the case planning process, and will allow for more effective matching of offenders and counselors. Notes