Flow cytometric analysis of blood leukocytes is currently used for both routine clinical measurements as well as for cutting edge research applications. This technology has enabled rapid and accurate determination of leukocyte antigens and quantitative analysis of leukocyte subsets, tests of leukocyte function, determination of the presence of antineutrophil and antilymphocyte antibodies in plasma and on cells, measurement of CD34+ hematpoietic stem cells in peripheral blood and bone marrow samples, measurement of apoptosis, and detection of virus-infected leukocytes. This review will focus on the use of the flow cytometer for investigations of blood leukocytes in transfusion medicine.