In this chapter we describe an approach to the analysis of human tumors that emphasizes the performance of multiple simultaneous measurements on each of several thousand cells in each tumor sample. This approach can be implemented using such technologies as flow cytometry, laser-scanning cytometry, or high-resolution image analysis. Here we will focus on flow cytometry studies. Before proceeding to methodological details, we would like to consider the theoretical and practical advantages of performing cell-based measurements (vis a vis gel or array based measurements, for example), and in particular, the advantages of performing multiple measurements per cell.