In recent years, air pollution due to the hazardous exhaust gases is a serious problem with harmful effects on plants, animals, human health, as well as causing global warming and climate change. Therefore, development of scalable and inexpensive nanosensors for highly sensitive, selective detection and monitoring of toxic gases is extremely important in improving environmental quality and protecting humans from over-exposure to such dangerous agents. One of the methods to improve the gas sensors performances is using the nanomaterials, such as nanoparticles, nano thin films, nanorods, and nanowires. Of those nanomaterials, nanowire structures are of interested due to their two quantum-confined directions while leaving one unconfined direction for electrical conduction, making them suitable for resistive-type gas sensor applications. In this content, I introduce the nanowire structures for gas sensor applications: form materials synthesis to practical devices. I hope you will find that this book provides useful information in the field of gas sensors.