The theory of swelling is reviewed in terms of basic concepts and simulation and impurity effects. The basic theory employs the formalism of chemical reaction rates. Efficiencies of voids, dislocations, and other extended defects for absorption of vacancies and interstitials have been derived. Phenomena such as void coalescence due to void growth and the effects of gas entrapped in voids have been modeled. Important questions, such as the dose dependence of swelling in various possible regimes, have been answered. The theory has been further developed to describe the effects associated with simulation of reactor swelling by charged particle bombardment. These include the temperature shift of swelling with changes in dose rate and the changes in swelling rate due to ion injection, the presence of nearby surfaces, and different modes of point defect generation. Impurities may have dramatic effects on swelling. Impurity trapping of point defects and some aspects of impurity segregation are understood theoretically. Improvements in the theory are possible, particularly in conjunction with experimental work. The more important areas are: additional mechanisms of impurity action, evolution of dislocation density, capture efficiencies of voids and other sinks, and the effects of gas other than in simply pressurizing cavities. From the theory, quantitative predictions of swelling have been made utilizing parameters obtained from micro-structural measurements on the same material at lower doses.