The structural features of tracheal diverticula are reviewed, and their taxonomic distribution summarized. Five possible functions are hypothesized for these structures: as an air reservoir, as an accessory site for gas exchange, to enhance buoyancy, to inflate the neck, and to modify the hiss acoustically. Three possible scenarios for the evolutionary origin of tracheal diverticula are discussed: through simplification of tracheal lungs, through subdivision of expanded tracheal membranes, and as de novo structures. The tracheal diverticula most likely function concomitantly to inflate the neck and acoustically modify the hiss. These structures presumably arose de novo, in either one or two evolutionary lineages.