As indicated in Chapter 33, flow cytometry has developed rapidly to provide a powerful means of characterizing complex cell populations, both in terms of quantitative analysis of functional cell-associated molecules, and, as is considered here in more detail, the simultaneous analysis of combinations of markers that can be used to identify functional subpopulations of cells. Many of the considerations discussed in the previous chapter are relevant, but issues particularly pertinent to this type of analysis relate to the independence of the markers used, both at the level of the labeling process and at the level of cytometric analysis.