More than 100 functionally distinct populations of lymphocytes can be identified in the peripheral blood of humans; each of these cell types undoubtedly has a unique role in the organization and effectiveness of an immune response. To distinguish specifically among these cells, however, it is necessary to measure simultaneously at least six to eight different T-cell surface antigens; additional functional correlations require even greater detection capability. For this purpose, flow cytometers capable of independently detecting as many as 12 different molecules now exist. Here we review the history and applications of this technology and discuss future directions.