Natural killer (NK) cells are a subset of lymphocytes that contribute to innate immunity through cytokine secretion and target cell lysis. NK cell function is regulated by a multiplicity of activating and inhibitory receptors. The advance in instrumentation for multi-color flow cytometry and the generation of specific mAbs for different epitopes related to phenotypic and functional parameters have facilitated our understanding of NK cell responses. Here, we provide protocols for flow cytometric evaluation of degranulation and cytokine production by human NK cells from peripheral blood at the single-cell level. In addition to offering insight into the regulation of human NK cell responses, these techniques are applicable to the assessment of various clinical conditions, including the diagnosis of immunodeficiency syndromes.