Amphibian embryos develop within the perivitelline space, which is bounded by the vitelline membrane and jelly envelopes, collectively called the egg capsule. Diffusion of oxygen through the capsule is described by the Fick equation, V̇o2 = Go2ΔPo2, in which V̇o2 is the oxygen consumption rate, Go2 is the oxygen conductance of the capsule, and ΔGo2 is the oxygen partial pressure diference across the capsule. The oxygen conductance of the capsule increases during development in parallel to V̇o2, and Po2 in the perivitelline space usually remains high enough to avoid limitation of V̇o2. The oxygen conductance of the capsule at the hatching stage is also matched with V̇o2 in species with small- to moderate-sized eggs, but there is evidence indicating that Go2 limits maximal V̇o2 in larger eggs. Compared to birds and reptiles, amphibians have a low allometric exponent for the relationship between V̇o2 and egg size, and there is a jump in V̇o2 at hatching. Trends toward incubation of larger eggs for longer periods, at lower temperatures, and out of water can be traced to problems of oxygen uptake. The oxygen conductance of the capsule in large eggs becomes constrained by a conflict between requirements for respiration (a large, thin capsule) and support (a small, thick capsule). The largest amphibian capsule that can supply sufficient oxygen contains somewhat less than 1 mL volume, so amphibian eggs must be small. The evolution of reptilian eggs involved functional abandonment of the capsule in favor of fibrous "shell membranes" in which respiratory gases could diffuse through air-filled channels. Loss of the capsule removed the constraints on Go2, V̇o2, and egg size. Respiration by an amphibian embryo in an egg mass is affected by its distance from the free environment and the oxygen uptake by other embryos in the mass. The respiratory role ofjelly is to separate the eggs, which reduces the "respiratory density" (volume-specific V̇o2) and therefore permits a larger number of embryos to occupy the mass. Nevertheless, the size of globular gelatinous egg masses is severely limited in aquatic and terrestrial environments. Limits to egg mass size can be extended by (1) spreading the eggs out to expose them more closely to the environment, either at oviposition or during development, (2) perforating the egg mass with channels in which water is convected, (3) depositing the eggs in foam, which supplies oxygen directly from the bubbles and augments the coeficient of oxygen diffusion, and (4) incorporating symbiotic algae into the mass. The vast diversity of amphibian reproductive mode ofers further opportunities to investigate the evolution of embryonic respiration within the physical constraints of the jelly capsule and the incubation environment.