Extract: Human blood contains hundreds of distinct cell types, which differ by the proteins they produce (including those expressed on the cell surface) and the functions they perform. Numerous clinical and research applications require monitoring of these blood cells for their presence or activity. The great demand for technologies to identify, quantify, and purify subsets of blood cells (particularly those involved in the immune response) has driven rapid technological advances in the field of fluorescence-activated flow cytometry. In this article, we describe the instrumentation, reagents, analysis, and applications for the most recent incarnation of flow cytometric technology: the 17-color flow cytometer.