Distributed Optical-Fibre Sensing (DOFS) is a technique which utilizes the very special properties of the optical fibre to make simultaneous measurements of both the spatial and temporal behaviour of a measurand field [1,2. As such, it provides an extra dimension in the measurement process, leading to finer monitoring and control, and to a new level of understanding, especially in regard to the behaviour of large structures. Thus we may expect to measure spatial distributions with a resolution 0.1 - im over a distance lOOm, to an accuracy —1%. The advantages of optical fibres for general measurement functions are well known: they rely on the insulating, dielectric, passivity of the medium, allied to the sensitivity of its optical propagation properties to xternal influences, over a broad range. DOFS takes advantage of two additional properties of the fibre: its one-dimensional nature, and its mechanical flexibility. With the aid of these, it becomes possible, in principle, to determine the value of a wanted measurand continuously as a function of position, along the length of a suitably-configured fibre, with arbitrarily large spatial resolution; the normal temporal variation is determined simultaneously, from the time-dependence of the signal. Such a facility as this offers many attractive possibilities for industrial and research application. The value of having access to the spatial/temporal behaviour of strain and temperature in, for example, large, critical structures such as dams, aircraft, space-craft, bridges, multi-storey buildings, electrical generators, boilers, chemical pressure vessels etc., are clear, from the points of view both of safety monitoring, and of improved understanding of behaviour under anomalous conditions. The flexibility of the fibre makes it relatively easy to install over the chosen measurement path and thus allows, unlike many other sensor systems, retrospective fitting. Furthermore, again in contradistinction to other sensor arrangements, DOFS offers a facility which is unique: there are no conventional techniques for acquiring the same information. This paper will summarize the principles of DOFS, will give examples of systems which have been studied, and give some indication as to what is in store for the future.