Flow cytometry can provide the clinician with a wealth of information on a given disease entity. It uses fluorescence and laser technology, thereby allowing characterization of a host of cell parameters and functions. Flow cytometry can be used to diagnose and prognose immunologic, infectious, and malignant disease, with minimal invasiveness. Any material within the reach of an aspiration needle is amenable to cytometric analysis. A qualified pathologist, well-trained technologist, and well-informed clinician all play crucial roles in the appropriate application of cytometric analysis of the clinical specimen. The wide variety of applications make flow cytometry a useful tool for all clinicians, regardless of practice location.