In this dissertation, a theoretical description of the operation of absorption and fluorescencebased optical chemical sensors employing planar multimode waveguides is developed. Using this theory, the design issues that should be taken into account in order to fabricate such sensor systems with an optimised performance are addressed and discussed in detail. Prototype sensor systems whose fabrication was based on these optimised design parameters are described and the experimental results of their performance are presented. The key issues dealt with in the dissertation are: 1. Development of a rigorous electromagnetic theory of operation of absorption-based optical chemical sensors employing multimode waveguides coated with a thin absorbing layer. 2. Application of this theory to the development of sensor systems with optimised performance. 3. Fabrication and experimental testing of a compact and portable absorption-based optical chemical sensor. 4. Development and experimental verification of a theory of radiation of fluorescent molecules embedded in a planar multilayer structure. 5. Application of this theory to the design of configurations with optimised fluorescence capture efficiency. Fabrication and experimental testing of these structures. 6. Development and testing of the experimental set-up facilitating the collection of a surfacegenerated fluorescence excited by a direct illumination.