Radiation from cosmic sources; naturally occurring radioactive materials, including radon and global fallout as it exists in the environment from the testing of nuclear explosive devices. The primary sources of radiological contamination include uranium mine sites, uranium mill tailings, uranium processing plants, nuclear weapons production facilities, nuclear testing laboratories, nuclear reactors, and associated fuel storage and radioactive waste storage and disposal facilities. The media that may be impacted by radiological contamination will vary from site to site and will be influenced by the source(s) of contamination and the method by which the contamination is released into the environment. The pathway by which a contaminant may spread into the surrounding environment is dependent upon the location and type of media that contains the contaminant. The chapter also presents an overview of the key concepts discussed in this book.