My interest in uterine natural killer (NK) cells began in the early 1990s, when I and others described the striking accumulation of NK cells in the uterine mucosa during the establishment of the placenta. Whilst working as a reproductive pathologist, I had become intrigued by pre-eclampsia, a mysterious syndrome that only occurs in pregnant women. It was the discovery of the killer cell immunoglobulin receptor (KIR) gene family by several groups, and in particular the study by Colonna and Samaridis in 1995, that provided the stimulus to understanding the link between NK cells and pre-eclampsia.