The strength characteristics and composition of bone substance are discussed in relation to its fracture characteristics. Procedures for estimating the toughness and fracture energy in directions parallel to and transverse to the direction of fibreing are introduced. Results are given for bovine, canine and anthropoid (rhesus monkey and human) bone, and are compared with other work where possible. Based on both light microscope and electron microscope examinations, a mode of fracture is propsed. The fracture of bone is also compared to the fracture of yet another directionally fibred substance-wood.