As T cells develop in the thymus, they undergo distinct changes in their morphology and expression of cell surface antigens. Phenotypic changes in mature peripheral T cells also occur subsequent to T-cell activation, resulting from engagement of the T-cell antigen receptor (TCR) or exposure to specific biochemical reagents. The ability to objectively measure such changes has been greatly enhanced by the combined techniques of immunofluorescence and flow cytometry (FCM). Using these approaches, it is possible to detect cell surface antigens by staining cells with specific antibodies that are tagged with different fluorochromes. Moreover, a variety of different cellular parameters can be monitored simultaneously on an individual cell. Whereas the initial investiture in this technology can be expensive, it provides a powerful and rapid means of assessing states of T-cell development and activation.