Over the world, researchers are interested, nowadays,in the resolution of the environmental problems such as the air pollution, the energy saving, and the development of different forms of renewable energy. In the same aim, I am interested during my research work in growth of different types of thin films and how to make better their properties if I need that.The present book is interested in the growth of iron oxide thin films by a simple technique, the spray pyrolysis, and using non-toxic and low cost precursors. The main aim was their use in the renewable energy conversion or sensors domains; which requires certain desired properties. Unfortunately, it wasn't the case in the beginning. So, the incorporation of the ruthenium in the layers composition was the good idea to make their properties better. The obtained Ru-alloyed iron oxide thin films have desired optical and electrical properties for their use in several domains. Furthermore, because of the ruthenium incorporation in the layers composition, interesting iron oxide phase needing higher energies to appear (Fe3O4), was obtained at lower annealing temperature; which is a good result.