Certain regions of the hypothalamus are important in aggression, but until recently, it has been difficult to specifically stimulate specific cell types within a mixed population of cells. Here, optogenetics is used to solve this specificity problem, finding that optogenetic stimulation of a subdivision within the ventromedial hypothalamus can elicit inappropriate attack behaviours in mice, but electrical stimulation does not produce the same result. Additional analysis of genetic and electrophysiological activity revealed overlapping neuronal subpopulations involved in fighting and mating, with potential competition between these behaviours, as neurons activated during aggression are inhibited during mating.