Human health as well as aquatic and terrestrial ecosystems are threatened from increa sing levels of environmental radiation of various sources, many of them of anthropoge nic causality: large areas of the former Soviet Union suffer from radioactive pollution, in particular after the Chemobyl accident; the increase in the incidence of UVB radiati on at the Earth's surface as a result of a progressive depletion of stratospheric ozone is a global problem that requires international concerted actions; in areas of former uranium mining the natural radiation level is substantially increased due to elevated radon levels; a growing portion of the population involved in air traffic is exposed to increased levels of natural radiation; and with the International Space Station an increasing number of astronauts will be exposed to the complex field of cosmic radiation. To estimate the corresponding risks, a better knowledge of the underlying radiobiological mechanisms at the molecular, cellular and system level is required. This book is the result of a multidisciplinary effort to discuss the current state of knowledge of the fundamental processes that result from interactions of environmental radiation -ionizing as well as UV radiation -with living matter and the existing radiati on protection concepts, and then to define future research work needed as fundamental information for the assessment of risks from increased levels of environmental radiation to human health and ecosystem balance. It comprises the key lectures and statements presented at the NATO Advanced Research Workshop.