This book gives a modern picture of the Earth's radiation environment and dynamics of radiation conditions in the heliosphere. The present monograph, unlike the reviews published earlier, treats the problem in self-contained form, in all its associations - from fundamental astrophysical, geophysical, and biological aspects to technical, engineering, aircraft and astronautical applications. The monograph includes a large amount of new data on the main sources of natural radiation hazard (terrestrial radiation belts, solar cosmic rays and galactic cosmic rays), accumulated during the last several decades of space research. As a result of the "information burst" in space physics, there are a lot of new interesting theoretical concepts, prediction models and ideas that deserve attention. The author gives an extensive bibliography, which covers impartially the main achievements, failures, problems and prospects in this field. The book will be helpful for a wide audience of space physicists, designers, engineers and other specialists in the practical cosmonautics (astronautics). It also will be relevant for a number of graduate courses on solar physics, geophysics, solar-terrestrial physics, and other branches of space research.