Gas sensor technology has advanced remarkably during the past few decades and is becoming an essential technology. Many gas sensors are now commercially available and researchers are making efforts, thanks to pioneering novel ideas, to develop next-generation gas sensors, having all the necessary requirements, such as small size, low production costs and power consumption. This book covers all gas sensor topics, a research field with an increasing interest in the last few years due to the demands of reliable, inexpensive and portable systems for environmental monitoring, indoor air quality, food quality control and many other applications. The goal of this book is to provide a critical assessment of the new trends in the gas sensor field, by describing the working principle and the applications related to the different types of sensors and paying attention to the recent impact of nanotechnology on the field.