Flow cytometry is a powerful tool, which uses lasers to analyze a wide range of different characteristics of cells. It is commonly used to determine the expression of cell surface markers and intracellular molecules to define cells into different populations using cell size, granularity, and fluorescently labeled antibodies. Thus, flow cytometry enables simultaneous and mutliparameter analysis of single cells. During the staining procedure, a single cell suspension is created for staining with flow cytometry antibodies for analysis on the flow cytometer. The staining methods from this technique can be used for different cell types by changing the surface marker targeted by the flow antibody, provided all antibodies are titrated prior to use, and are chosen with knowledge of the density of surface molecule for detection and brightness of fluorochrome to guide antibody selection. This chapter’s protocol has been designed specifically for detection of human CD4+ T cell subsets defining naïve and memory subpopulations by surface marker phenotyping.