Natural killer (NK) cells have long been considered to be a homogenous population of innate lymphocytes with limited phenotypic and functional diversity. However, recent findings have revealed that these cells comprise a large number of distinct populations with diverse characteristics. Some of these characteristics may relate to their developmental origin, and others represent differences in differentiation that are influenced by factors such as tissue localization and imprints by viral infections. In this Review, we provide a comprehensive overview of the emerging knowledge about the development, differentiation and function of human NK cell populations, with a particular focus on NK cells in peripheral tissues.