The use of Raman spectroscopy in probing the deformation mechanisms of cellulose fibres (regenerated and natural), and two natural cellulose composite systems (wood and paper) is described. It is shown that during tensile deformation the 1095 cm(-1) Raman band, corresponding to the stretching of the cellulose ring structure, shifts towards a lower wavenumber due to molecular deformation. By analysing a number of fibres with different microstructures this shift is shown to be invaluable in understanding the micromechanisms of deformation in these materials. Moreover, the rate of Raman band shift is shown to be invariant with stress for all fibre types, consistent with a fibre microstructure based on a modified series aggregate model. In the composite systems, such as wood and paper, it is shown that the stress-induced Raman band shift in the cellulose gives an important insight into their local deformation micromechanics.