NK cell development requires IL-15, which is "trans-presented" to IL-15Rβγ on NK cells by IL-15Rα on other cells. In this study, we report that different levels of IL-15 trans-presentation are required for different NK cell developmental events to reach full maturation status. Because the IL-15Rα intracellular domain has the capacity to recruit signaling molecules, we generated knockin and transgenic (Tg) mice that lack the intracellular domain to assess the role of the IL-15 trans-presentation level independent of the function of this domain. The level of IL-15Rα on various cells of these mice follows the order WT > Tg6 > knockin > Tg1 ≥ knockout. Bone marrow (BM)-derived dendritic cells prepared from these mice induced Stat5 phosphorylation in NK cells. The level of phospho-Stat5 correlated with the level of IL-15Rα on BMDCs, thus offering the opportunity to study quantitative effects of IL-15 trans-presentation on NK cell development in vivo. We found that NK cell homeostasis, mature NK cell differentiation, and acquisition of Ly49 receptor and effector functions require different levels of IL-15 trans-presentation input to achieve full status. All NK cell developmental events examined were quantitatively regulated by the IL-15Rα level of BM-derived and radiation-resistant accessory cells, but not by IL-15Rα of NK cells. We also found that IL-15Rα of radiation-resistant cells was more potent than IL-15Rα of BM-derived accessory cells in support of stage 2 to stage 3 splenic mNK differentiation. In summary, each examined developmental event required a particular level of IL-15 trans-presentation by accessory cells.