Gas sensor techlogy has advanced remarkably during past few decades and has become one of the indispensable techlogies for modern society. Varieties of gas sensors are commercially available and, using invative ideas, efforts are being made to develop gas sensors of next generation having very small size with very low power consumption. The ultimate model for this is probably given by sensory organs of our own body, which are implanted finely and work well with a very modest amount of energy. In order to achieve this goal, it is essential that various aspects of gas sensors are seriously considered. These include understanding of gas sensing mechanisms, development of new materials and methods to synthesise them into selective sensors, invations in nastructured materials, measurement methods, microfabrication of sensors, exploring intelligent sensing system, etc. This book examines these issues pertaining to chemiresistive gas sensors.