Identification of natural killer (NK) cell subsets has gained attention with the recent discovery that mature mouse NK cells comprise two distinct stages. Delineation of the stages is performed using the markers CD27 and CD11b on gated NK cells. The significance of this finding is underpinned by recent discoveries that mature human NK cells can also be discriminated by differential expression of CD27. This chapter will describe the methods required for the purification of lymphocytes from blood and other organs and the delineation of NK cell subsets by flow cytometry.