Sensors which rely on the external modulation of the properties of an optical fibre (intrinsic sensors) are receiving much attention since they can be made extremely sensitive, and can be used for distributed measurements. Distributed sensing provides some particularly exciting prospects for acoustic, magnetic and electric field monitoring. To date, however, the great majority of experimental and commercial fibre sensors employ telecommunications-grade fibres, largely as a result of their ready availability. Not only does this policy frequently lead to a design compromise, but in some cases makes the performance marginal or untenable as a result of excessive environmental sensitivity. Despite this, little attention has been given to the design of special sensor fibres with enhanced (or depressed) sensitivity to specific measurands. The position is somewhat better with respect to fibres designed to eliminate sensor polarisation problems (e.g. polar isation-maintaining fibres), but even here further work is required to provide the performance demanded.