Flow cytometry immunophenotyping (FCI) of hematolymphoid specimens is a powerful tool in clinical medicine aiding in the accurate diagnosis and subclassification of acute and chronic leukemias, non-Hodgkin lymphomas, myelodysplasia, and other hematolymphoid neoplastic processes. Multiple flow cytometric strategies are used to evaluate hematolymphoid populations, including identification of neoplastic populations with aberrant immunophenotypes, abnormal maturation patterns, monotypic kappa/lambda light chain expression, restricted V-beta expression, and abnormal light scatter properties. In this chapter, we present a general approach to analyze hematolymphoid populations. Specimen handling, gating strategies, and appropriate antibody panels for flow cytometric detection of abnormal hematologic populations are presented.