The ceca, intestinal outpocketings of the gut, are described, classified by types, and their occurrence surveyed across the Order Aves. Correlation between cecal size and systematic position is weak except among closely related species. With many exceptions, herbivores and omnivores tend to have large ceca, insectivores and carnivores are variable, and piscivores and graminivores have small ceca. Although important progress has been made in recent years, especially through the use of wild birds under natural (or quasi-natural) conditions rather than studying domestic species in captivity, much remains to be learned about cecal functioning. Research on periodic changes in galliform and anseriform cecal size in response to dietary alterations is discussed. Studies demonstrating cellulose digestion and fermentation in ceca, and their utilization and absorption of water, nitrogenous compounds, and other nutrients are reviewed. We also note disease-causing organisms that may be found in ceca. The avian cecum is a multi-purpose organ, with the potential to act in many different ways - and depending on the species involved, its cecal morphology, and ecological conditions, cecal functioning can be efficient and vitally important to a bird's physiology, especially during periods of stress. Received 14 Feb. 1994, accepted 2 June 1994.