Flow cytometry is an invaluable tool for studying lymphocyte biology. In transplantation, flow cytometry represents the most sensitive method for demonstrating the binding of HLA antibodies to cellular targets. Referred to as the flow cytometric crossmatch (FCXM), the implementation of this method has revolutionized HLA antibody assessments and facilitated increased allograft survival among highly sensitized recipients. Here we describe the methods for performing the FCXM and discuss its critical aspects, as well as the presentation of data and interpretation of results.