Ressler, Burgess, and Douglas (1988) consider a murder sexual if at least one of the following is involved: the victim is found totally or partially naked; the genitals are exposed; the body is found in a sexually explicit position; an object has been inserted into a body cavity; there is evidence of sexual contact; or there is evidence of substitutive sexual activity or of sadistic sexual fantasies. The main obstacle to labeling a homicide as sexual is the failure of some police officers to use these features of a crime scene as evidence of the sexual nature of a homicide, as well as the lack of specific definitions for sexual murder in criminal codes. In Canada between 1974 and 1986, there were 305 sexual murders (approximately 23 murders annually). In the United States between 1991 and 1995, sexual murders accounted for only 0.9 percent of all murders; however, there was no difference in the rate of sexual murder in the two countries, since the United States has a higher overall murder rate than Canada. Serial sexual murderers account for only a small proportion of all sexual murders. An overview of the biological, psychological, and sociological theories of sexual murder, notes that empirical evidence for any of the theories is limited. The review of research on sexual murderers advises that few studies of nonserial sexual murderers have included a comparison group composed of other types of sexual offenders. Only three studies have compared nonserial sexual murderers to sexual aggressors against women. The chapter concludes with an overview of the Montreal Study of Sexual Murderers of Women.