The rate theory of cavity growth is applied to study the effects of helium gas on cavity swelling. The variation of swelling with temperature is emphasized. (1) Expressions are derived showing that the primary effect of the helium is in pressurizing cavities and that a secondary effect is in altering the microstructural sink strengths. (2) Recent experimental data on swelling of a pure stainless steel-type alloy under dual-nickel and helium-ion bombardment are interpreted. Helium-free, helium-coimplanted, and helium-preimplanted swelling results can be explained by the theory. It is necessary to account for the partitioning of the helium to dislocations as well as to cavities in order to explain the experimental results for helium coimplantation. (3) Model studies for physically reasonable parameters reveal the importance of the helium/displacements per atom ratio.