This review explores the current state of knowledge about sexual homicide, which is often based on uncertainties and contradictory assumptions. In the introduction we examine some of the methodological issues, such as low prevalence rates and operationalisation problems, that contribute to the situation regarding the existing knowledge base. The research investigating the key types of sexual homicide and the possible characteristics distinguishing sexual killers from others types of sexual offenders are outlined in order to provide the reader with the background to the aims of this systematic review. Additionally, throughout the paper, we go back to the various methodological complications so that in the method section of this review we can note the ways researchers have tried to overcome them.While there are issues that make compiling accurate figures challenging (see later), the studies looking at the prevalence of sexual killing consistently show that it is a relatively rare phenomenon with estimates ranging from approximately 6% of all homicides in the UK (Beech, Fisher, & Ward, 2005) to 4% of homicides in Canada (Roberts & Grossman, 1993) and 1% of homicides in the USA (Chan & Heide, 2009). Further, it seems that in North America, the number of homicides assessed as sexual in nature has been declining (James & Proulx, 2014). Although estimated to be rare, the statistics from the UK note that in June 2012 in the UK there were 6320 men serving a conviction for murder or manslaughter, and 840 of these men killed with an apparent or admitted sexual element (Planning and Analysis Group, National Offender Management Service, NOMS, personal communication, August 2013). This means that out of all men detained for murder or manslaughter in the UK prison population, 13.3% are assessed as sexual murderers and this number represents an increase from the May 2003 estimate of 6% (Beech et al., 2005).The topic of sexual homicide has been extensively examined for the last two decades (Chan & Heide, 2009). Despite this examination, the many methodological complications encountered by researchers, as discussed in this review, hamper our ability to better understand and draw conclusions about this crime and its perpetrators. Specifically, both the low prevalence rates noted above and the difficulties in accessing detailed descriptions of the offense inevitably mean that small sample sizes are usually used in studies (Porter, Woodworth, Earle, Drugge, & Boer, 2003). Accounting for the different role that killing can play in relation to sexual behaviour could help to improve our understanding of this crime (Stefanska, Carter, Higgs, Bishopp, & Beech, 2015). It is worth noting that while the term sexual killing, murder and homicide tend to be used interchangeably, they generally encompass cases of murder, manslaughter as well as cases of diminished responsibility, with or without a hospital order.