Solar energy semiconductor photocatalysis has long been considered to be the best solution to various kinds of energy and environmental problems. During the past decades, the solar energy semiconductor photocatalysis has been extensively applied in photocatalytic pollutant degradation, H 2 evolution, and CO 2 reduction. In this chapter, history of solar energy photocatalysis was first carefully discussed. Subsequently, fundamental principles of solar energy photocatalysis, including basic mechanisms, thermodynamic and dynamics requirements for solar energy photocatalysis, were thoroughly clarified. Then, design principles, classifications, and modification strategies of semiconductor photocatalysts, and development approaches of novel semiconductor photocatalysts were systematically addressed. In addition, various kinds of photocatalytic processes and evaluation methods of solar energy photocatalysis were also summarized in detail. Finally, the scope of this book was highlighted. It is expected that this chapter would provide the general understanding of photocatalysis and basic design and application rules of solar energy semiconductor photocatalysis.