Increasing environmental pollution has led to a growing demand for highly sensitive, cheap, user- friendly devices to monitor air pollutants to protect the health of human beings. Surface Acoustic Wave (SAW) based sensors can be produced relatively inexpensively, have low power consumption and are capable of sensing low analyte gas concentrations. They can be easily interfaced with electronics, making them portable devices for multi- spot outdoor measurements. Since 1979 they have been successfully utilised for gas sensing applications and today are used in a wide range of sensor fields, mainly in chemical and bio-sensing. Improvements in their performance can be obtained by the application of suitable intermediate and gas sensitive layers deposited on various piezoelectric substrates. This book presents novel Layered SAW based gas sensors with indium oxide gas sensitive layers, which were utilised for ozone and hydrogen sensing applications. The reader can also find a description of the fabrication of SAW devices, sensor layers' material characterisation and the gas sensing mechanism of semiconductor metal oxide based gas sensors.