Flow cytometry is the primary tool for phenotyping leukemias and related conditions. With the ever increasing numbers of antibodies commercially available, the ability to study and understand leukemias, myelodysplastic syndromes, and the myelodysplastic/myeloproliferative diseases is improving. Although the data generated by flow cytometry are not comprehensive enough to completely subtype leukemias or myelodysplastic syndromes into their myriad divisions, this wealth of information does provide phenotype, reproducible enumeration of blasts, certain prognostic information, and it can reveal the presence of cell populations with aberrant antigen profiles. An important drawback to flow cytometry as it is performed today is the inability to look at the cells that mark with the antibody panels used. As classifications of leukemia and related conditions evolve, flow cytometry continues to answer many of the questions asked and to provide critical information reliably and quickly.