Fibre-optic sensing techniques for application in polymer electrolyte fuel cells (PEMFC) are presented in this thesis. Temperature, relative humidity (RH) and air-water two-phase flow sensors are developed and demonstrated based on optical fibre Bragg gratings (FBG). Bragg gratings offer the following characteristics that warrant their development for application in PEMFCs: small size, environmental compatibility and the possibility of multiplexed multi-parameter sensing. Contributions of this work are in novel sensor development and implementation strategies. Important installation design considerations include the sensor proximity to the catalyst layer, sensor strain relief and minimal bending of the fibre. With these considerations, the dynamic and steady-state performance of FBG temperature sensors distributed throughout the flow-field of a single cell PEMFC was validated with a co-located micro-thermocouple. In the development of FBGs for in situ measurement of relative humidity, a polyimide-coated FBG based RH sensor is presented with significantly improved response time and sensitivity over previously reported designs. The RH inside a PEMFC under transient operating conditions is monitored. Step increases in current induce significantly larger increases in RH near the outlet than near the inlet of the cell, and associated transients within the fuel cell are found on a time scale approaching the sensor response time. Finally, to complete the suite of FBG sensors for water management in PEMFCs, an evanescent field based FBG sensor embedded in a microchannel for the measurement of two-phase flow dynamics is presented. Using high speed video for validation, it is established that the novel sensor enables the measurement of droplet average velocity and size in flow regimes representative of an operating fuel cell.