Gases are the key components in many industrial and domestic activities. The specific demand for gas detection and monitoring has emerged particularly as the awareness of the requirements to protect the environment has grown. Gas sensors can find applications in numerous fields, such as in fire detectors, leakage detectors, controllers of ventilation in cars and planes, and alarm devices warning the overcoming of threshold concentration values of hazardous gases. The current research to enhance both functions in gas sensors is an indispensable tool to create new technologies which are compatible with sustainable society. Further advancements are desired strongly across the world. This book collects chapters from a number of experts in the gas sensing field with different research backgrounds. It can serve as a useful reference for researchers to have a comprehensive understanding of the development of gas sensors and the trends for future investigation.