Crimes relating to sexual behaviour account for only 1 per cent of all indictable crimes but for half a century sex offenders have been the subject of a disproportionate amount of psychiatric interest and enquiry. The belief, that there is a link between specific mental malfunction and sex offending, as distinct from other forms of offending appears to have wide acceptance. In 1972 the Royal College of Psychiatrists appeared sufficiently confident of this view to ask candidates aspiring to membership of the College to “Give an account of the psychopathology of rape”. Like incest and buggery, rape is an act, a deed, an item of behaviour. To speak of the aetiology, treatment and prognosis of such acts is to misapply terminology and to confuse criminal behaviour with mental disorder. The problem is not simply one of semantic niceties for the sloppy use of medical language in this controversial area can lead jurists and policy-makers to carry expectations that lack any firm psychiatric basis. This contribution will attempt to review some legal and psychiatric aspects of the more serious crimes.