The surrounding of cancer cells (“tumor microenvironment) is a critical determinant of treatment responsiveness. Especially in many solid malignancies, the malformed tumor vasculature fails to meet the demand for oxygen. Oxygen deprivation renders many standard-of-care anti-cancer treatments significantly less effective compared to normoxic conditions. Notably, oxygen content is only one of many physico-chemical parameters that define the tumor environment. We are currently elucidating whether and how actively modifying the tumor microenvironment could enhance effectiveness of cancer therapies.