The most commonly used personality-based model is that developed by Megargee (1977). Ten clusters of offender profiles are identified, including operational definitions of profile characteristics. These 10 types were found to differ significantly on 75 of 86 non-MMPI (Minnesota Multiphasic Personality Inventory) variables, and reliability was demonstrated. Numerous validation studies have been conducted to assess the applicability of this system to different populations, with mixed results. The behavior-based classification system used by the Federal Bureau of Prisons is largely based on current offense and prior criminal record and recommends appropriate levels of custody and security. The behavior-based model implemented by the California Department of Corrections derives a score based on the inmate's background (five social factors), institutional behavior, and sentence length. The National Institute of Corrections' classification system is a composite of the models used in California and the Federal Bureau of Prisons. Despite the wide use of classification procedures, rigorous cross-validation studies have not yet been conducted to determine the predictive value of many of these models. Where studies have been conducted, the research has generally been disappointing. 29-item annotated bibliography.