The mechanisms suggested to cause caldera resurgence can be categorized as (1) viscous rebound of magma, (2) regional detumescence, and (3) pressure against the base of the caldera block due to magmatic vesiculation and convection or addition of new magma. The critical observation that resurgence takes of the order of 103-105 years insists that the causative mechanism be regulated by a highly viscous (e.g., ~1022 Pa s) structural member of the crust. The relatively low viscosity magma and a purely elastic crust cannot be important in achieving this time scale. The most likely mechanism is the coupled effect of regional detumescence and magmatic vesiculation. Detumescence compresses the magma chamber, squeezing magma upward against and through the caldera block. Vesiculation also compresses the chamber and tends to dome the caldera block. The times of both detumescence and doming of the caldera block by magmatic pressure are probably inversely proportional to the size of the magma chamber, but the rate of cooling of the chamber is proportional to its size. Because solidificaiton spreads viscous relaxation over a wide region of the crust and the crust and curtails resurgence, there is likely to be a minimum volume of magma that can undergo resurgent, doming. It is difficult to be exact about this critical size until more precise models have been developed.