In 1978 an experiment showed that 488 nm laser radiation can induce self-written refractive index gratings in optical fibres when launched into the core. No real excitement pervaded the scientific community until efficient side-writing was shown in 1989. In contrast to self-written gratings that only operate at the writing wavelength, side-written gratings can have any periodicity longer than the writing wavelength. Driven by the many important applications in telecom and sensor fields, great progress has since been made both in the writing-technology and in the research for new photosensitive materials. This thesis describes the work carried out at the University of Southampton to develop new materials and post-fabrication techniques for enhanced photosensitivity and to understand the basic mechanisms behind such a phenomenon. A low-loss, highly-photosensitive and telecom-compatible fibre has been fabricated by using tin as the only dopant of silica. Its physical mechanism was studied and a model was proposed to explain the results. A conventional post-fabrication method (hydrogen loading), used to improve photosensitivity, showed that gratings written in tin-doped fibres exhibit extraordinarily high thermal stability. Photosensitive fibres for high-temperature sensing were also developed; gratings written with a 248 nm laser were shown to survive for 30 minutes at a record temperature of 850oC. Finally the development of a post-fabrication technique to increase the photosensitivity was tested on many fibres and a possible explanation of its physical mechanism was found.