Currently, the mainstays of treatment for most forms of cancer are chemotherapy, radiation, and surgery. Unfortunately, many patients fail these conventional treatments, and it is difficult for physicians to understand how much tumor could be affected (or killed eventually) by the therapy in any given patient. In just the past half decade, we and our colleagues have clearly illustrated the need for a physical oncology–based approach to treating cancer, by viewing cancer as a combination of physical and biological problems, rather than a purely biological one. Fundamental physical processes in tumors, such as the movement of nutrients, delivery of drug molecules, and exchange of mechanical forces between cancer cells and the surrounding tissue, depend on many biological processes, including the growth of blood vessels, the formation of fibrous collagen matrix, and cancer cell proliferation. These biological processes exert physical forces on the cancer cells and ultimately influence the growth patterns of the tumor and its response to treatment. By developing mathematical models to describe these growth patterns and tumor responses to therapies, we have recast the biological problem into a bioengineering one.