Flow cytometry (FC) plays a very important role in diagnosis, subclassification, and posttreatment monitoring of hematologic neoplasms [1–26]. FC results allow to choose the most appropriate further testing [such as fluorescence in situ hybridization (FISH) or polymerase chain reaction (PCR)] to establish the definite diagnosis and to further characterize the malignant process in a timely manner. Multiparameter FC measures simultaneously several surface and/or intracytoplasmic markers on a single cell, allowing for accurate phenotypic characterization of analyzed population(s). While no single marker permits a definite lineage assignment, analysis with panels of antibodies allows for separation of hematologic tumors into very precise subtypes with different prognosis and treatment requirements, as defined by the current World Health Organization classification of hematopoietic and lymphoid tumors [27]. FC analysis can precisely differentiate between B- and T-cell malignancies, and between mature (peripheral) and precursor tumors, and determine the myeloid or lymphoid lineage among the latter. Availability of new markers (antibodies) and new fluorochromes, and improvement in instrumentation not only increases the accuracy of FC studies and allows for identification of a minute abnormal population among the majority of benign cells, but also expands FC applications into disorders such as myelodysplastic syndromes (MDSs), paroxysmal nocturnal hemoglobinuria (PNH), or myeloproliferative neoplasms (MPNs) [9,18,26,28–34].