The global environment that surrounds contains small amounts of radioactive elements or radionuclides that are derived from primordial, secondary, cosmogenic, and anthropogenic sources. Radionuclides in the air, soil, water, and rocks that make up the Earth’s geosphere and atmosphere can be transferred into the biosphere by many organisms and bioaccumulated in the food chain. Most of the radioactivity to which we are exposed daily comes from background natural sources commonly occurring in our surrounding environment and the buildings in which we live. Traditionally radionuclides have been separated into three categories or types: primordial and secondary, cosmogenic, and anthropogenic. The chapter discusses the three types of radionuclide sources and highlights some of the more important examples. Primordial radionuclides, such as uranium, thorium, and certain isotopes of potassium, have very long lifetimes and were produced at or before the creation of planet Earth. Secondary radionuclides are derived through radioactive decay of the long-lived primordial parent nuclides.