Recognition that the environment affects human health goes back to the time of the Hippocratic physicians, when ills were thought to be due to interactions among the sick person’s internal “humors,” his lifestyle, and his environment. Recently we have become aware of the harmful effects that treating illness imposes on the environment, creating, in turn, new health hazards. This month’s issue examines the health/environment feedback loop and, specifically, the duty of physicians to understand how humans and their environment can harm each other.