Natural killer (NK) cells are a subset of innate lymphoid cells that are present throughout the body and serve a myriad of functions in human health and disease. NK cells derive from multipotent hematopoietic progenitor cells that reside in the bone marrow, respond to local cues allowing them to exit the marrow, and subsequently undergo a series of developmental steps resulting in NK cell lineage differentiation and functional maturation. Although NK cells ostensibly can complete their development within the bone marrow, transitional cells along the NK cell developmental spectrum are enriched in blood and peripheral tissues, which can further instruct NK cells to adopt tissue-specific phenotypes. Thus, terminal steps of NK cell development likely occur throughout the body. Further, due to the dynamic interplay between cell intrinsic capacity for adaptation and cues from the tissue microenvironments, NK cell development is highly malleable to meet the changing needs of the body throughout life. However, this also means that in the setting of disease, such as cancer, NK cell development can be impacted, sometimes deleteriously, the consequences of which are a subject of ongoing investigation. This chapter describes the processes, cellular intermediates, locations, and regulation of human NK cell development.