Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron--hole pairs, which generate free radicals able to undergo secondary reactions. Its comprehension has been made possible ever since the discovery of water electrolysis by means of titanium dioxide. Commercial application of the process is called Advanced Oxidation Process (AOP). There are several methods of achieving AOP's, that can but do not necessarily involve TiO2 or even the use of UV. Generally the defining factor is the production and use of the hydroxide anion. This new and important book gathers the latest research from around the globe in the study of photocatalysis and highlights such topics as: water and wastewater treatment by heterogeneous photocatalysis, bactericial and antiviral effects of photocatalysts, and others.