Suicide with accompanying homicide is frightening and ill understood.To raise professional awareness of its complexities and difficulties and identify areas for developing research.A Crime in Mind seminar was held in London in December 2018, with four expert presentations and discussion. This paper draws on that seminar and supplementary literature.Homicide/suicide is very difficult to predict and thus prevent. Victims and perpetrators may have a dependency relationship. Better training, especially of general practitioners, may increase the likelihood of detecting signs of despair and delusional ideas. Psychiatrists should be more alert to fixed delusions and homicidal thoughts. Individual assessment and management alone is unlikely to be sufficient. Public health strategies are likely to be important too. Weapons control is vital. Employers of people with great personal responsibility and special access to potentially lethal tools, such as airline pilots, and perhaps clinicians, should have regular, compulsory, mental health checks.Six points of clinical importance emerged, including lowering the threshold for diagnosing delusional disorder and the establishment of anti-violence clinics.