No generally accepted theory of sex offender behavior exists at this point. As clinical experience and research findings interact within an evolving theoretical framework, the picture of what is important, as well as what is possible to know about a sex offender, is rapidly changing. It is vital that mental health professionals, the legal system, social service agencies, and other consumers and providers of sex offender assessments be aware of both what is possible to learn about an offender as well as the limitations on that knowledge and its application. Prominent theories that influence the assessment of offenders in North America and Europe are presented and examined. Uses and limitations of each theory are discussed. Several trends emerge which could influence the future of sex offender assessment, treatment, management, and policy.