Flow cytometry provides a rapid and accurate means of analyzing individual cells in suspension. Several parameters can be detected simultaneously and analyzed to provide information on the relationship between them. Fluorescently labeled cells can give quantitative data on specific target molecules and their distribution in the cell population. Moreover, subpopulations of cells with specific attributes can be discriminated and physically sorted for further development. These advantages have given flow cytometry the potential to be applied at all stages of process development, from cell line selection to the development of monitoring and control strategies for animal cell bioreactor operation.