AbstractThe development of models of fuel-element performance requires, among other things, a detailed, quantitative understanding of fuel swelling and fission gas release. The current state of knowledge on these phenomena is reviewed. It is shown that gas bubbles play an important role in both swelling and gas release, that a knowledge of fuel operating history is essential to quantitative predictions, and that the migration of cesium will greatly influence local swelling rates. Finally, an indication is given of the areas toward which future studies should be directed.