Flow cytometry (FC) plays an important role in the diagnosis and classification of hematological disorders and can analyze a wide range of diagnostic samples. It is used in both diagnostic and follow-up testing, and is increasingly important in the detection of very small residual disease populations, i. e., minimal residual disease (MRD). Modern flow cytometers have the capability to analyze several thousands of cells per second and to identify different cell populations by evaluating the expression of specific antigens (markers) along with light scatter properties of the cells. FC can precisely differentiate between B‑ and T‑cell malignancies, between mature and precursor tumors, and among the latter, determine the myeloid or lymphatic lineage.