Physical processing or pretreatment of lignocellulosics concerns the ultrastructural modification of materials such as wood, straw and bagasse. The substrates produced can be subsequently converted by chemicals. The various pretreatment options will be discussed in the light of the ultrastructural, polymeric and chemical modifications that are obtained. The processes can be classified as follows: (i) steam; (ii) aqueous; and (iii) organosolvolysis treatments. All of these have their antecedents in the thermomechanical processes developed by the pulp and paper or fibreboard industries. Sequential application of thermomechanical technology leads to fractionation of the substrate into the major polymeric fractions: cellulose, hemicellulose and lignin in varying degrees of modification. A number of pretreatment concepts are now at a commercial scale and are being applied to produce foodstuffs from lignocellulosics for use by ruminant animals. The same techniques are being piloted in the energy and chemicals from lignocellulosics field.