This literature review shows that the term 'sexual offender' encompasses a heterogeneous group of offenders and crimes. The following perspectives have been used to investigate the descriptive characteristics of rapists and child molesters: sociological, legal, psychiatric, psychometric, and physiological/behavioral. The crucial variables identified have been cultural influences, offense variables, victim characteristics, and offender characteristics. Classification schemes for rapists have identified four groupings: those acting out of aggression, those motivated by anger toward women, those who are sadistic, and those who have a broader antisocial lifestyle and criminal history. Classifications of child molesters have identified one type with an exclusive and longstanding preference for children, a second type which has regressed from adult levels, and a third type which is psychopathic with poor social skills. None of the schemes has been evaluated for predictive validity or generalization. Refining existing classifications and empirically generating new systems are two strategies for advancing the classification of this offender population. Research conducted at the Massachusetts Treatment Center demonstrates these strategies. Multiple approaches and methods as well as research conducted by many investigators will be needed in this task. Tables and a list of 196 references are included.