A number of new optical techniques for both thein vivo identification of blood analytes and the measurement of physical properties of interest to the clinician have recently been developed. These optical transducers are based on glass or plastic fibres, fractions of a millimetre (100–250 μm) in diameter, as found in fibre-optic communication systems. These sensors offer a number of advantages for physiological applications over their conventional electrical counterpart, the most important of which are: their non-electrical nature; the operating optical signal is not subject to electrical interference; the fibre allows a high degree of mechanical flexibility combined with very small size and low-cost disposable construction; materials suitable for long-term implantation, e.g. plastics, may be used in their construction. Optical sensors fall into three categories: (a) the photometric or bare-ended fibre; (b) the physical sensor which is used to measure temperature and pressure; and (c) the chemical sensors or ‘optrodes’ based on the immobilisation of a reagent phase on the end of the fibre. Clinical sensors based on fibre-optics, although clearly attractive in concept, are mostly in the developmental stage. However, the devices developed to date already illustrate the potential systems that are possible with this expanding technology.