To achieve successful solutions to the problems resulting from local, distant and global radioactive fallout after nuclear explosions and accidents and to achieve successful retrospective analyses of the radiation conditions from recent observations, certain information is needed: the distribution of the exposure dose rate in the atmosphere and in a country; the distribution of radionuclides in natural environments and the nuclide composition of the radioactive fallout; the features of formation of the aerosol particle-carriers of the radioactivity and of the nuclide distribution of the particles of different sizes formed under different conditions; the processes involved in the migration of radioactive products in different zones and environments; the external and internal effects of nuclear radiation on human beings. This monograph is devoted to a number of these problems, namely, to studies of the radioactive fallout composition, the formation of the aerosol particles that transport the radioactive products and to the analysis of the external radiation doses resulting from nuclear explosions and/or accidents. Problems of restoration and rehabilitation of contaminated land areas are also touched upon in the monograph. To solve such problems one requires knowledge of the mobility of radionuclides, an understanding of their uptake by plants, their transportation within the food chain and finally their uptake by animal and/or human organisms. The results of many years of study of radioactive fallout from atmospheric and underground nuclear explosions and accidents are summarized in this book. It is intended for various specialists - geophysicists, ecologists, health experts and inspectors, as well as those who are concerned with radioactive contamination of natural environments.