Flow cytometers are technically sophisticated instruments that utilize pr- ciples of physics, chemistry, engineering, digital electronics, optics, analog devices, laser technology, and biology to measure the light scattering and fluor- cent characteristics of particles. These instruments have become invaluable tools in the area of cancer research, cell biology, and clinical diagnostics for analyzing a multitude of cellular and subcellular particles. Undoubtedly, flow cytometers have become widely used because they provide a very unique means of exam- ing biological particles. Large quantities of cells/particles are individually a- lyzed, and it is typical for flow cytometers to quantitatively process thousands of individual particles in a matter of seconds. This a powerful analytic feat parti- larly if one relates it to the time required to examine several thousand individual cells using a microscope. This leaves little doubt regarding why the field of flow cytometry has advanced so rapidly since the first flow cytometers were commerically available in the 1970s. Today, flow cytometers are common in both large and small institutions. Current flow cytometers have evolved with advances in computers and microelectronics into fast and reliable instruments. The science of flow cytometry has also benefitted from rapid growth in the fields of biochemistry and immunology over the past decade. A vast array of fluorochrome-con- gated antibodies and other reagents are now commercially available for pr- ing structures and monitoring function in human cells as as in many other species.