During the past 30 years, we have seen flow cytometry (FCM) emerge from being a research tool requiring a group of engineers, an optical bench, and a darkened room to a benchtop flow cytometer that is used routinely in a clinical setting. The flow cytometer is to cell biology what the UV-visible spectrophotometer is to solution spectroscopy. Several events have contributed to the development of this technology to where it is now an indispensable tool in the diagnosis and clinical monitoring of disease. Many cell types are now under intense investigation. The clinical application of FCM to lymphocyte subset immunophenotyping in the leukemias and lymphomas was responsible for the early development of clinical FCM.