Immunophenotyping in the clinical laboratory is emerging as an advantageous way to separate and classify leukemic malignancies. Immunophenotyping involves the use of flow cytometers and immunofluorescence in order to achieve great sensitivity and specificity for malignant cells. A basic understanding of components of the flow cytometer and how it works is necessary to understand immunophenotyping. Monoclonal antibodies specific to the malignant cells of question play an essential part in this technique. Various fluorescent dyes and cell panels also must be incorporated into the system. Analysis is done and statistics are plotted on dot plots that can be read by the CLS to give helpful insight into the etiology of disease process. Immunophenotyping is a very powerful tool that has the ability to revolutionize the clinical laboratory setting. The CLS working in hematology must become aware of and comfortable with this methodology.