The mechanical properties of collagen fibers primarily depend on the formation of head to tail Schiff base cross-links between end-overlapped collagen molecules within the fiber induced by the enzyme lysyl oxidase. Inhibition of these cross-links results in the complete loss of mechanical strength of the fiber. During maturation these initial divalent cross-links react further with molecules in register from an adjacent fiber forming stable trivalent cross-links and further increasing its mechanical strength. This system of cross-linking is well established and exists throughout the animal kingdom from sponges to man, but there remain a number of unidentified cross-links known to be present in some tissues. In addition, there are some unusual cross-links in certain invertebrates. The nature of the collagen cross-linking is tissue specific rather than species specific and depends on the extent of hydroxylation of both the telopeptide and triple helical lysines involved in the cross-link and on the rate of collagen metabolism. The cross-link profile of collagen fibers therefore varies considerably within and between tissues, for example in different bones. Recent studies indicate that pyrrole cross-links rather than pyridinoline cross-links correlate with mechanical strength of avian bones. The profile can change between normal loading and extreme exercise and these differences appear to relate to their particular function, but further studies to identify whether a particular cross-link is responsible remain to be carried out.Following maturation the low turnover of collagen allows the non-enzymic random accumulation of glucose oxidation products, some of which form intermolecular cross-links, ultimately rendering the fiber too stiff for normal function. The significance of the major glycation cross-link, believed to be glucosepane, remains to be confirmed. The successful use of inhibitors of this glycation reaction and of specific glycation cross-link breakers should lead to a reduction in this deleterious effect in both aging and diabetes mellitus.The high mechanical strength and resistance to heat and bacterial degradation of collagen fibers has been utilized industrially. Additional chemical cross-linking in vitro has been employed historically to attain specific mechanical and thermal properties, for example tanning skin to leather, and more recently in medical and cosmetic products with low cytotoxic effects. The resultant increases in denaturation temperature have recently been correlated with reduced water content of the fiber.