Description:National Council on Radiation Protection and Measurements Report No. 76. The purpose of this Report is to review the current status of the application of radionuclide transport models from the point of discharge to the environment to the point of intake by man. This process, called radiological assessment, begins by defining the quantity of radionuclides that are released and enter the environment. Uncertainties in the measurements or estimates of the source term are passed on to the assessment calculations. This Report does not specifically address the development of source terms, however it should be borne in mind that important information about source terms must be available in order to determine which radionuclide transport models to use in an assessment. This information should incorporate the proximity of different points of release, the time distribution of release, the quantity and species of radionuclides released, and the chemical and physical form of the radionuclides. In the description of the models that follow, sufficient information is presented to allow the user to determine which of the source term conditions listed above should be considered.