Oxford University Press, 2013. All Rights Reserved. This chapter discusses the steps needed to perform an assessment of the internal dose resulting from exposure to radionuclides. It provides information about where to obtain guidance on radionuclides currently recognized by the International Commission on Radiological Protection (ICRP), and how to apply the compartmental physiological models adopted in the ICRP publications. Occupational exposures as well as some examples of environmental exposures to the general population are examined.