Flow cytometers are instruments that can quantify fluorescence intensity data and provide unique information about cell populations. Significant advances have been made in terms of calibration reagents, evaluation of the quality and limitations of the monoclonal antibodies, standardized sample preparation, and data analysis to ensure interlaboratory comparability and reproducibility. Efforts to standardize quantitative fluorescence intensity measurements impact current clinical flow cytometry applications (i.e., immunophenotyping), but also emerging technologies, such as microarray assays, which also require calibration of fluorescence intensity across different platforms.