In last two centuries, air quality got worse due to several factors related to the modern human development. The presence of pollutants in atmosphere makes people passive agents subject to many types of diseases. To monitor prejudicial gases, the development of fast, sensitive, and selective sensors is a rewarding field that matches with the acquired knowledge of semiconducting surfaces, in particular, of semiconducting metal oxides. This chapter discusses the principles and structures of advanced materials employed in the development of gas sensors, including electronic and the nanoscale approaches, and the strategies to improve them.