Archosaurs and mammals exhibit skeletal pneumaticity, where bone is infilled by air-filled soft tissues. Some theropod dinosaurs possess extensively pneumatic skulls in which many of the individual bones are hollowed out by diverticula of three main cranial sinus systems: the paranasal, suborbital, and tympanic sinuses. Computed tomography (CT scanning) permits detailed study of the internal morphology of cranial sinuses. But only a few theropod specimens have yet been subjected to this type of analysis. We present CT scans of the remarkably preserved and disarticulated skull bones of the long-snouted tyrannosaurid theropod Alioramus. These scans indicate that Alioramus has extensive cranial pneumaticity, with pneumatic sinuses invading the maxilla, lacrimal, jugal, squamosal, quadrate, palatine, ectopterygoid, and surangular. Pneumaticity is not present, however, in the nasal, postorbital, quadratojugal, pterygoid, or angular. Comparisons between Alioramus and other theropods (most importantly the closely related Tyrannosaurus) show that the cranial sinuses of Alioramus are modified to fill the long-snouted skull of this taxon, and that Alioramus has an extreme degree of cranial pneumaticity compared to other theropods, which may be the result of the juvenile status of the specimen, a difference in feeding style between Alioramus and other theropods, or passive processes. Based on these comparisons, we provide a revised terminology of cranial pneumatic structures and review the distribution, variation, and evolution of cranial pneumaticity within theropod dinosaurs. This review illustrates that most theropods possess a common “groundplan” in which the maxilla and lacrimal are pneumatized, and that various theropods modify this groundplan by pneumatizing numerous other bones of the skull. Tyrannosaurids are very pneumatic compared to other theropods, particularly in the development of extensive ectopterygoid, quadrate, and palatine sinuses, as well as a pneumatic invasion into the surangular. Tyrannosauroids seem to retain many cranial sinuses, such as the jugal and nasal recesses, which are primitive for coelurosaurs but lost or apomorphically modified in taxa more closely related to birds.