In this book we have discussed the preparation and characterization of CdO thin films using RF reactive magnetron sputtering technique. The deposition parameters such as oxygen partial pressure, substrate temperature and film thickness are optimized for producing good quality films. Systematic characterization of as deposited and annealed films has been discussed from the crystal structure, surface morphology, film composition, optical and electrical properties. The films prepared under optimized conditions are tested for gas sensing characteristics towards ammonia gas.