Summary: Our understanding of human natural killer (NK) cell development lags far behind that of human B- or T-cell development. Much of our recent knowledge of this incomplete picture comes from experimental animal models that have aided in identifying fundamental in vivo processes, including those controlling NK cell homeostasis, self-tolerance, and the generation of a diverse NK cell repertoire. However, it has been difficult to fully understand the mechanistic details of NK cell development in humans, primarily because the in vivo cellular intermediates and microenvironments of this developmental pathway have remained elusive. Although there is general consensus that NK cell development occurs primarily within the bone marrow (BM), recent data implicate secondary lymphoid tissues as principal sites of NK cell development in humans. The strongest evidence stems from the observation that the newly described stages of human NK cell development are naturally and selectively enriched within lymph nodes and tonsils compared with blood and BM. In the current review, we provide an overview of these recent findings and discuss these in the context of existing tenets in the field of lymphocyte development.