Sex offenders are a growing population within both the prison system and the community mental health system. Social workers provide the majority of mental health services in this country, and therefore the profession needs to better understand this population, including the theories of etiology, methods of assessment, treatment options, evaluation strategies, and demographic characteristics for this group. This article is the first of a two-part series on sex offenders focusing primarily on the clinical issues of this population so that their needs are more accessible to social workers.