The discovery in recent years of several well preserved and partially articulated skeletons of Tyrannosaurus rex has provided the impetus for a study of sexual dimorphism in dinosaurs. The fifteen significant specimens of Tyrannosaurus rex discovered to date may be divided into “robust” and “gracile” morphotypes. Examinations of extant crocodiles and birds show a link between musculature of sexual organs and sexual dimorphism of osteoskeletons. Analogous differences in articulated dinosaur remains point to a method of determining the gender of individual dinosaur skeletons and the presence of an intromittent organ in at least some species. For theropods, and particularly for Tyrannosaurus rex, the larger and more robust individuals are almost certainly female.