The atomic bomb and other studies have established with certainty that moderate-to-high doses of radiation cause many types of solid cancer and leukemia. Moving down the dose range to the vicinity of 100-200 mSv, the risks become fuzzy and then unknown at low doses on the order of 10-20 mSv. Nor have low-dose experimental studies provided definitive answers: some have suggested there may be adverse biological effects in the range of 5-50 mSv, while others support a "no risk" interpretation. Epidemiologic data contain intrinsic "noise" (variation by known and unknown factors related to genetics, lifestyle, other environmental exposures, sociodemographics, diagnostic accuracy, etc.) so are generally too insensitive to provide compelling answers in the low-dose range. However, there have been recent provocative reports regarding risk from relatively low-dose occupational and medical radiation exposures that warrant careful consideration. Summaries of the largest studies with low-dose or low dose-rate radiation exposure provide suggestive evidence of risk for solid cancer and stronger evidence for leukemia risk. Recently, interest in health endpoints other than cancer also has risen sharply, in particular the degree of cardiovascular and cataract risk following doses under 1 Sv. Data regarding cardiovascular disease are limited and fuzzy, with suggestions of inconsistencies, and the risk at low doses is essentially unknown. The evidence of cataract risk after low dose-rate exposures among those conducting interventional medical radiological procedures is becoming strong. The magnitude of radiation impacts on human health requires fuller documentation, especially for low-dose or low dose-rate exposures. From the epidemiologic vantage point, this will require longer observation of existing irradiated cohorts and development of new informative cohorts, improved accuracy in dose assessments, more attention to confounding variables, and more biosamples from irradiated groups to enable translational radiobiological studies. Introduction of Radiation Impacts on Human Health (Video 2:02, http://links.lww.com/HP/A35).