This book presents the study and development of sensing systems for the remote detection of gases with the use of microstructured optical fibres of the hollow-core type (HC-PCF) and the use of signal processing techniques usually associated with absorption spectroscopy. Photonic Crystal Fibres (PCF) arose as an exceptionally interesting player in the field of gas sensing since they promote the creation of short and direct interaction paths between light and gas and can be tuned to address any specific gas. Thus, their principle of operation and main optical properties were studied and described. The diffusion time of gases inside of microstructured fibres is a subject of special importance. As a consequence of its theoretical study and experimental analysis, fruitful results for the planning of a new configuration for a sensing head were achieved. Light coupling between standard optical fibres and hollow-core photonic crystal fibres is a practical issue that was also evaluated in the aim of the work here presented. Wavelength Modulation Spectroscopy is also presented and described in this book as it is a powerful and sensitive technique suitable for gas detection.