Hormesis is a dose-response phenomenon in which a low dose of a toxin has the opposite effect on a biological system than a high dose of the same toxin. It is generally characterized as toxic effects that are beneficial at low doses and harmful at high doses. There is some ambiguity in the more precise definition of the term, however, because some speak strictly of low-dose stimulation of biological endpoints (for example, immune system strengthening), whereas others also use it to refer to low-dose inhibition of biological endpoints (such as tumor formation). Hormesis has long been marginalized in medical and environmental fields. A growing body of evidence suggesting hormetic effects across a wide range of biological organisms and systems, however, has brought increased credibility to the topic. The implications of hormesis are potentially huge, especially in terms of risk assessment policies and research paradigms. Skepticism and controversy persist...