Keratinized epidermal appendages have mechanical properties which are typical of those expected for composite materials with a filament-matrix texture. At 100% relative humidity the matrix is highly hydrated, mechanically weak and exhibits viscoelastic behaviour. As the water content is reduced the matrix becomes progressively stiffer until at 0% relative humidity its properties approach those of the filaments. In the normally encountered range of relative humidities the advantages of such a texture are that stress is evenly distributed over the filaments, which constitute the load-bearing elements, thus preventing the propagation of cracks from local imperfections. Based on studies of synthetic filament-matrix composites potentialities for the adaptation of the mechanical properties to specific functions are to be found in variation of the properties of the filaments and the matrix and also of the cross-linking. Since keratin is a filament-matrix composite at the molecular level the great diversity of molecular species presumably has its origin in such adaptation. Superimposed on filament and matrix composition and properties are the variables of filament orientation and proportion and there is abundant evidence that many of the subtler properties of keratinized appendages stem from variations in these factors.