BACKGROUND/AIMS: Wing skeletons of bats are uniquely specialized for flight, reflecting both evolutionary history and the need to maintain structural integrity while generating aerodynamic forces. METHODS: We analyzed the anatomical structure of bat wing skeletons in the context of scaling patterns relative to other mammals, material properties and the mechanical function of the wing bones during flight. RESULTS: Compared with nonvolant mammals, the bones of the bat forelimb are elongated, even after correcting for shared phylogenetic history. Bats have consistently larger-diameter bones in the forelimb than do nonvolant mammals but significantly narrower hindlimb bones. Mineralization in the cortical bone of wings is lower than in the long bones of other adult mammals, with a proximodistal gradient of decreasing mineralization. The distal phalanges have only a small amount of mineralized tissue underlying the articular cartilage. Loads required to elicit a 10% length deflection in the wing bones of Glossophaga soricina varied approximately 50-fold along the wing and flexural stiffness nearly 200-fold. Commensurate with low mineralization and flexural stiffness, bat bones experience extraordinarily high bending strains during flight. CONCLUSION: Bat limb skeletons share features with other mammals and possess specialized characteristics, mostly related to the mechanical demands of flight.