Continuous monitoring of physiological parameters such as pH and oxygen (O2) are of great importance in determining the health status of a patient. Arterial blood gas analysis is a current clinical method used to measure pH, PCO2, PO2, and the concentration of variety of ions, typically with blood withdrawn from an artery. The need for robust, and a rapidly responding technology to enable bed-side monitoring has driven considerable efforts to produce better sensor devices. Optical sensing systems have experienced rapid growth, with drivers including low-cost optical fibres, and the availability of miniature optical set-ups (light sources, detectors, etc.). Herein, polymer-based optical fibre sensors for pH and O2 sensing were developed. The pH and/or oxygen reporters were immobilised at the end of an optical fibre by photo-polymerisation, and their performance in measuring pH and O2 concentration investigated. pH sensing was based on fluorescence detection using single excitation/single emission (Chapter 2), and single excitation/dual emission (Chapter 3). O2 sensing was based on the luminescence quenching of metalloporphyrins by oxygen (Chapter 4). In the last chapter, the in vivo applicability of an O2 sensor was investigated by measuring O2 level changes inside an ex vivo lung.