Flow Cytometry is the primary tool in most immunology laboratories for the detailed characterization of lymphocytes and lymphocyte function. The ability to resolve highly complex populations of cells, interrogate the cells one-by-one, make multiple measurements on each cell, and then separate the cells viably for further study make this technology unique and indispensable. Our laboratory has long been involved in pushing this technology forward; we have perhaps the most sophisticated flow cytometer in the world, currently capable of making 20 different measurements on each cell. This platform has proven invaluable in dissecting immune responses to vaccines and viruses. In addition, we are developing novel algorithms for the automated analysis of the highly complex data that results from these experiments.