AbstractAlpha keratin fibers (hairs, wools, quills, and other mammalian appendages) together with fibroin fibers such as silks and spiders webs are all highly extensible fibrous proteins for which the mechanical properties are of primary importance both to the animal from which they originate and their ultimate application by man. Similarly, the collagens are highly inextensible fibrous proteins, which form the major component of mammalian skin and connecting structures such as tendons. All these fibrous proteins are biological polymers of polypeptide chains for which the mechanical and allied physical properties, such as water absorption, relate to both their macrostructure and their molecular and near‐molecular structure. Because of both their commercial application and their relatively complex structure at the molecular and near‐molecular level, interpretation of the physical properties of α‐keratin fibers represents the main component of this presentation. The mechanical properties of α‐keratin fibers are primarily related to the two components of the elongated cortical cells, the highly ordered intermediate filaments (microfibrils) which contain the α‐helices, and the matrix in which the intermediate filaments are embedded. The matrix consists of globular proteins plus water, the content of the latter being dependent on the fibers environment. The Extended Two‐Phase Model (ETPM) has been developed and results in a detailed coverage of the bulk mechanical properties of α‐helical fibers in terms of their known molecular and near‐molecular structure. The inextensible protein fibers, the collagens and fibroins, are also briefly discussed in terms of the relationship between mechanical properties and the structure of these fibers. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 489–507, 2002