Protein materials constitute Nature's most intriguing material concepts, leading to multi-functional and stimuli responsive materials. Such materials often feature a characteristic hierarchical design, which is characterized by structural features starting at nanoscale. Here we review recent studies of deformation of collagen, Nature's most abundant structural protein material forming the basis of bone, tendon and skin. We have discovered that a specific nanostructural design with molecular lengths of 200 nm leads to the strongest possible ultra-structure that is still capable of dissipating large amounts of energy before fracture occurs, maximizing the toughness of the material [M.J. Buehler, Proceedings of the National Academy of Sciences USA 103 (2006) 12285]. The analysis explains prevalent molecular length scales observed in tendon, bone and the eye's cornea, and explains how molecular properties influence the deformation and fracture mechanics of tissues. (C) 2007 Elsevier B.V. All rights reserved.