Flow cytometry (measurement of cells as they flow by a detector) has been available for analysis and sorting a variety of cell types in fluid suspension since the late 1970s. Flow cytometers use focused laser light to illuminate cells as they pass the laser beam, one at a time, in a fine fluid stream. Light scattered by the cells and light emitted by fluorescent dyes attached to cells of interest are analyzed by several detectors and processed by a computer. Cells may be distinguished and selected on the basis of size and shape as well as by the presence of many different molecules inside and on the surface of the cells. The power of flow cytometry over other techniques is: