This book describes green photocatalysts and their diverse applications in the fields of environmental sciences and energy. It especially takes a closer look at the removal of air and water pollutants, the generation of hydrogen, photo fuel cells, electrophotocatalysts, solar energy conversions, and green biophotocatalysts. Furthermore it also discusses on the role of catalysts along with their chemical reactions, challenges, past developments and directions for further research on photocatalysts. It includes recent developments of quantum dots (QDs) and photocatalytic applications of QDs such as carbon materials like carbon and graphene based QDs, metal, metal sulfide and metal oxide based QDs as well as a detailed review on various types of templates used for the preparation of porous g-C3N4 and its applications in detail. This is done with special reference to dye degradation, reduction of hexavalent Cr, and reduction of CO2 and for the evolution of H2 photocatalytically. This book offers an intriguing and useful guide for a broad readership in various fields of catalysis, material sciences, environment and energy.