Genetic, epigenetic and functional diversity between tumor cells can affect tissue dynamics, tumor growth and treatment dynamics in many ways. I will talk about two examples of diversity. First, I will describe a setup in which non-cell-autonomous interactions led to maintenance of diversity in in vivo breast cancer tumor growth. Second, I will describe an approach to describe the basic features of cancer stem cell and differentiated cancer cell dynamics, which can be used to analyze tumor burden from individual patient trajectories, and to make predictions for treatment continuation, e.g. in leukemia.