Immunophenotyping by flow cytometry is a robust and highly complex technology used in the enumeration and characterization of leukocytes, including normal lymphocyte subsets and hematologic neoplasms. Samples consist of peripheral blood and many times irreplaceable samples, such as bone marrow and fresh tissue. These samples are collected, transported, prepared, analyzed, and interpreted, resulting in diagnostic and prognostic information. Such information is critical to treatment decisions for patients. In order to obtain accurate and reproducible results, it is essential to have optimized and standardized procedures, rigorous quality control, and assurance programs encompassing preanalytic, analytic, and postanalytic processes.