Cancerous tumors are diverse communities of cells, which in addition to cancer cells may contain vascular cells, immune cells, and connective tissue-generating fibroblasts. Cancer cells can manipulate other cell types to help the cancer grow and spread, while cancer therapies can manipulate other cell types to prevent cancer growth or even kill cancer cells. As researchers learn more about how cancer cells interact with other cell types and components of the tumor microenvironment, they not only learn more about how cancer progresses, but also discover new opportunities to treat cancer.