This review covers the development of the understanding of the deformation micromechanics of both synthetic and natural polymeric fibres using spectroscopic and X-ray diffraction techniques. The concept of fibres as composites, where hard and stiff phases are combined with softer polymeric materials is also discussed. Starting with the first discoveries on the molecular orientation and morphology of polymeric fibres, the widely used concepts of uniform stress and strain are examined for the analysis of fibre deformation. The use of advanced techniques such as Raman and infrared spectroscopies to follow molecular deformation in both rigid-rod (e.g. PpPTA, PBO, PBT, polyethylene) and natural (e.g. cellulose, collagen, silk, chitin) polymer fibres is presented. A clear distinction between fibres that have structures that are subjected to uniform stress or strain is presented, with the evidence that is detected from the response of the molecules (by Raman spectroscopy) and the crystalline fraction (by X-ray diffraction). It is suggested that natural fibres, such as cellulose, silk and others, may have different types of microstructures that are subjected to a uniform strain, which could have potentially led to incorrect determinations of crystal moduli. It is also demonstrated that the Raman and X-ray techniques have been influential on our development of fibres, and have shown that the morphology plays a critical role in mechanical properties. In addition to this, the use of X-ray diffraction using microfocus synchrotron sources is also reviewed. This approach allows a more complete picture of both molecular and crystalline deformations to be developed, and with the advent of nanocomposites it is shown that a combination of the two techniques will be vital for our understanding of their exploitation in technological applications. (c) 2006 Elsevier Ltd. All rights reserved. ; ISI Document Delivery No.: 152CD Times Cited: 15 Cited Reference Count: 173 Young, R. J. Eichhorn, S. J.