This chapter outlines the key environmental and metabolic pathways of radionuclides relevant for animals and animal products after NREs. The different methods used to quantify the transfer of radionuclides between relevant environmental pathways are also described. Detailed information is provided for the most important radionuclides that contaminate animals and animal products, namely radiocaesium, radioiodine and radiostrontium. Observed effects on agricultural and game animals after two NREs are also briefly described.