Natural killer (NK) cells comprise 5-20% of peripheral blood mononuclear cells (PBMC) in humans. In addition to their fundamental roles in the defense against viral infections and tumor surveillance, NK cells help shape adaptive immune responses through their production of cytokines. NK cells are traditionally identified as CD3(neg), CD14(neg), CD19(neg) lymphocytes expressing CD56. Using a combination of markers that includes CD56 and CD7 greatly increases the ability to define the phenotype and function of NK cell subsets. Two key markers of NK cell function are the production of IFNγ and the release of cytotoxic granules measured by the expression of CD107a. Here we describe a method to assess IFNγ and CD107a expression in NK cells following stimulation with target cells or cytokines. This method can be used to assess the general functional capacity of NK cells in peripheral blood mononuclear cells from a wide range of study participants.