After reviewing laws that have imposed controls on mentally ill persons and research on the relationship between sexuality and mental impairment, the paper describes the substantive law dealing with sexual offenses such as rape and indecent assault. Problems that arise when the victim is mentally impaired are discussed, particularly the capacity to consent and whether the offender must be aware of the impairment. The paper recommends against a general prohibition on people having consensual sexual relations with persons with impaired mental functioning, but believes that sexual relations between mentally impaired persons and those responsible for their care and supervision should continue to be prohibited. A discussion of procedure issues considers whether reporting sexual crimes committed against mentally impaired individuals should be legally mandated, whether specialist investigative resources are needed, and whether prosecution should give priority to cases involving mentally impaired victims. A mentally impaired victim's competency to testify and the admissibility of sexual history evidence are explored. A summary of issues and proposed reforms is provided.