The vertebrae of sauropod dinosaurs are characterized by complex architecture involving laminae, fossae, and internal chambers of various shapes and sizes. These structures are interpreted as osteological correlates of an intricate system of air sacs and pneumatic diverticula similar to that of birds. In basal sauropods pneumatic features are limited to fossae. Camerae and camellae are internalized pneumatic chambers independently acquired in neosauropods and some Chinese forms. The polycamerate and camellate vertebrae of higher neosauropods are characterized by internal pneumatic chambers of considerable complexity. The independent acquisition of these derived morphologies in Mamenchisaurus, derived diplodocids, and most titanosauriforms is correlated with increasing size and neck length.The presacral vertebrae of basal sauropods were probably pneumatized by diverticula of cervical air sacs similar to those of birds. Although pneumatic characters in sauropods are most extensive and complex in presacral vertebrae, the sacrum was also pneumatized in most neosauropods. Pneumatization of the proximal caudal vertebrae was achieved independently in diplodocids and titanosaurids. In birds, the synsacrum is pneumatized via abdominal air sacs which function primarily in lung ventilation. The presence of pneumatized sacral and caudal vertebrae in neosauropods indicates that abdominal air sacs were probably present in at least some sauropods.