Gas sensors are a part of transducers and monitoring and measurement systems which, in addition to sensors, contain signal conversion and display elements. The main function of gas sensors is to convert a concentration of the analysed gas into an electrical or any other signal that can be registered and visualised. The most commonly used are semiconductor, electrochemical and optical (IR) sensors. In sensors of the first two types, adsorption of the analysed mixture component causes changes in sensors' electrical properties; sensors of the third type register changes in the optical density of the analysed gas mixture at a specific wavelength. The most important characteristics of gas sensors are selectivity (or cross-sensitivity) for a particular component, component concentration detection limits, and response time (sensor response to changes in concentration of the component).