Homicide-suicide entails a homicide followed by the perpetrator’s suicide within one week. The incidence of homicide-suicide in the US was 0.23% per 100,000 in 2013 (about 5% of all US homicides). In Western Europe and other low violence countries, such as Japan, homicide-suicides make up a much higher proportion of all homicides. Subtypes are filicidal, spousal (including jealous and declining health subtypes), familial, and extrafamilial homicide-suicide. Spousal homicide-suicides are the most common, yet extrafamilial homicide-suicides receive the most media attention, despite their rarity. Related phenomena include mass murder, victim-precipitated suicide (“suicide by cop”), politically motivated homicide-suicide, and suicide in violent offenders. We review several conceptual models of the etiology of homicide-suicide, namely developmental, dynamic, biological, and cognitive models, and draw implications from the current state of knowledge about homicide-suicide.