A strain sensor comprises an optical waveguide having a plurality of reflecting structure (Bragg grating) along its length. Each structure reflects light at a different characteristic wavelength (&lgr;1 to &lgr;n+1) which changes in dependence on a change of physical length of at least part of the reflecting structure. The reflectivity of reflecting structures which reflect at characteristic wavelengths which are adjacent to each other (&lgr;1 and &lgr;2 or &lgr;n and &lgr;n+1) are configured to be different such that the intensity of light reflected from adjacent structures can be used to discriminate between them.