NK cells respond to infected and transformed cells using germline-encoded NK receptors (NKRs). Collectively, these NKRs combine to form distinct repertoires that tune the NK cell response. To provide a framework for understanding how perturbations in expression of NKRs influence disease pathogenesis, we defined the phenotypic heterogeneity of NK cells in 22 healthy individuals (ages 21-62; M=10, F=12), including 5 sets of monozygotic twins. Using mass cytometry, surface expression of 38 markers specific for NKRs and lineage markers to identify B cells, T cells and myeloid cells were examined. KIR and HLA class I genotypes were determined by Luminex, and KIR gene content was further assessed using pyrosequencing. NK cell populations, as defined by inhibitory NKRs, were highly concordant between twins (R2 = 0.90) in comparison to unrelated individuals (R2 = 0.66), indicating that the inhibitory repertoire is highly genetically determined. However, when analyzing all twenty-five NKRs, including activating receptors, the concordance in twins (R2=0.34) and in unrelated individuals (R2=0.11) was greatly decreased, indicating significant environmental influence in determining the overall NK cell repertoire. Overall, these analyses reveal tremendous diversity within the NK repertoire that is both genetically and environmentally determined, and offer the ability to examine the functional capacity of NK cells with far greater resolution.