This book deals with the preparation, characterization and photocatalytic activity in gas and aqueous media of undoped and doped TiO2 nanopowders and thin films. Nanocrystalline anatase of 25 nm size with a 1 at. % dopants were successfully prepared by the cost effective sol gel method as powders, and applied on glass substrates with spin coating method. Techniques such thermal analyses, XRD, SEM, DLS, XPS, UV-Vis and Raman spectroscopy were used in order to characterize the obtained materials from a morphological, optical and structural point of view. Degussa TiO2 P25 nanopowder was used as a reference. Photodegradation experiments indicated that NO2 and MO solution can be degraded in gas phase and in the aqueous TiO2 suspensions respectively. A comparison of the photocatalytic efficiency of undoped and doped TiO2 immobilized onto substrates versus catalysts in suspended form is reported. Photocatalysis is a very promising method to face most of the problems connected with the increasing environmental pollution, and titanium dioxide, in its anatase phase, is one of the most investigated photocatalytic materials and it's perfectly compatible with every day human activities.