Natural killer (NK) cells were originally defined as mediators of spontaneous cytotoxicity against virus-infected and tumor cells. In human peripheral blood, the majority of NK cells can mediate cell lysis mainly through perforin and granzymes. It has been recently shown, however, that the majority of NK cells in human secondary lymphoid organs are primarily immunoregulatory by secreting cytokines immediately after activation and do not express perforin and granzymes. Because lymph nodes (LN) harbor 40% and peripheral blood only 2% of all lymphocytes in humans, this newly characterized NK cell compartment in LN and related tissues probably outnumbers perforin⁺cytolytic NK cells in our body. Although human NK cell biology has so far mainly studied peripheral blood NK cells, we have lately focused on human NK cells harbored in lymphoid tissues and identified procedures for their optimal isolation as well as their phenotypic and functional characterization.