Fibre-optic sensor technology provides a means of performing sensing functions in hostile environments, such as explosive or high electromagnetic-interferenceprone areas, whilst also offering advantages in terms of convenience of application. For these reasons fibre-optic sensors are currently of great interest to large sections of the industrial and military community. The paper presents a selection of the sensor systems upon which Plessey Research has worked in recent years and which illustrate this wide interest. The fibre-optic gas sensor and the fibre-optic distributed temperature sensor are examples of sensors with general industrial interest. The analogue spectral-filtering sensor system and the fibreoptic gyroscope are of interest to the aerospace industry. The hydrophone has potential naval applications whilst fibre-optic evanescent field sensors have an attraction to medical and industrial users. For each of these sensor types the principle of operation and the demonstrated performance are described and the current state of development commented on.