Optical-fibre development to date has concentrated on their use in systems for telecommunications and data transfer. The realisation that optical fibres could also be used for sensors and sensing systems has increased rapidly in recent years. Judged by the large number of publications in this area the research effort is extensive, but few systems have yet been put to practical use. A critical review of recent developments is therefore useful at this stage. Several examples of fibre-optic sensors are given, where their selection has been dependent on the system showing some practical viability, and data having been obtained relevant to operation in a real (as distinct from purely laboratory) environment. Devices ranging from simple fibre-optic switches and pollution monitoring equipment, to highly sensitive acoustic, magnetic and rotation sensing systems are emerging, where an understanding of modal and polarisation effects in single-mode fibres is necessary. At present each sensor system tends to be aimed at specific application areas, where the form of acceptance by the user will vary. It is emphasised that such developments require engineering inputs at the earliest possible stage. The use of optical fibres for direct actuation of valves and sensors has now closed the optical control loop.