Extraordinary advances in molecular biology and biotechnology have led to the development of a vast number of therapeutic anti-cancer agents. To reach cancer cells in a tumor, a blood-borne therapeutic molecule, particle, or cell must make its way into the blood vessels of the tumor and across the vessel wall into the interstitium, which it then must migrate through. Unfortunately, tumors often develop in ways that hinder these steps. The goal of research in this area is to analyze each of these steps experimentally and theoretically and integrate the resulting information into a unified theoretical framework. This paradigm of analysis and synthesis has fostered a better understanding of physiological barriers in solid tumors and aided in the development of novel strategies to exploit and/or overcome these barriers for improved cancer detection and treatment.