Sea birds with long incubation periods are identified, together with the features of their incubation physiology which distinguish them from birds in general. Most sea birds with prolonged incubation are members of the order Procellariformes. The majority of Pelecaniformes and Charadniformes with long incubation periods are tropical species. The total amount of water lost from the egg during incubation is a similar fraction of the initial egg weight in sea birds with prolonged incubation as in other birds. The oxygen consumption of the newly hatched chick is similarly related to the chick weight, regardless of the duration of incubation. Within the constraints imposed by these similarities, sea birds with prolonged incubation display a number of adaptations. The daily rate of water loss from the egg, the water vapor conductance of the egg shell, and the total functional pore area of the egg are all relatively low in sea birds with prolonged incubation. The eggs of sea birds with long incubation times are large in relation to the size of the adult bird in the two species that have been studied, the high energy content of the egg is paralleled by the greater total amount of oxygen consumed during incubation. However, the growth of the embryo is relatively slow in at least one sea bird with a long incubation time so that prolonged incubation probably entails a comparatively high allocation of energy resources to maintenance requirements. In some species with prolonged incubation, the interval between pipping and hatching is also long and it appears to be a period of great physiological importance. Ecologically prolonged incubation is associated with either pelagic feeding habits or a tropical environment, both factors may be related to food supply.