This paper describes a task performed for the U. S. Nuclear Regulatory Commission (NRC), consisting of using post-Chernobyl data from Norway to verify or find areas for possible improvement in the chronic exposure pathway models utilized in the NRC’s program for probabilistic risk analysis, level 3, of the MELCOR accident consequence code system (MACCS), developed at Sandia National Laboratories, Albuquerque, New Mexico. Because of unfortunate combinations of weather conditions, the levels of Chernobyl fallout in parts of Norway were quite high, with large areas contaminated to more than 100 kBq/m2 of radioactive cesium. Approximately 6% of the total amount of radioactive cesium released from Chernobyl is deposited on Norwegian territory, according to a countrywide survey performed by the Norwegian National Institute for Radiation Hygiene. Accordingly, a very large monitoring effort was carried out in Norway, and some of the results of this effort have provided important new insights into the ways in which radioactive cesium behaves in the environment. In addition to collection and evaluation of post-Chernobyl monitoring results, some experiments were also performed as part of the task. Some experiments performed pre-Chernobyl were also relevant, and some conclusions could be drawn from these. All the long-term exposure pathways routinely treated by MACCS were considered. The Chernobyl accident brought no new insights to the cloudshine exposure pathway, but understanding of the groundshine, soil-grass-milk, soil-grass-beef, and the freshwater exposure pathways was considerably improved. Much new valuable information on exposure pathways not routinely included in MACCS has also been gathered, but this aspect is not discussed in this paper. In most connections, the data available show the models and data in MACCS to be appropriate. A few areas where the data indicate that the MACCS approach is inadequate are, however, also pointed out in the paper. This concerns in particular root uptake to grass from soil and the freshwater exposure pathways. Both of these areas ought to be revised. It is also pointed out that MACCS’ inability in the present version to distinguish between chemical forms of cesium with different bioavailability may lead to conservative results. The task is limited to radioactive cesium, which proved to be by far the most important post-Chernobyl radionuclide in the Norwegian area.