Solar energy is the mother of almost all forms of energy available to the civilized world and the sustainable development of the mankind should depend on efficient and massive utilization of sunlight. Photochemical processes have been intensively investigated for this purpose. In particular, the photoinduced electron transfers occurring at the semiconductor surface are the key process of solar photosynthetic and photocatalytic transformation. This phenomenon has been extensively investigated for the environmental purification of water and air and the solar energy storage through solar fuel synthesis. Metal oxides such as TiO2, WO3, and Fe2O3 that consists of earth-abundant elements are the most practical base materials for such applications. Despite their popularity as solar conversion materials, breakthroughs in materials development have yet to be achieved for practical applications. A variety of approaches have been investigated to modify the base metal oxides using diverse inorganic and organic materials. The heterojunctions built at the interface of metal oxide reduce the charge recombination or enhance the interfacial charge transfer to achieve the higher conversion efficiency. In this talk, the general principles of this technology will be introduced and some research examples will be discussed in view of how various modifications of the semiconductor interface affect the solar conversion activities and mechanisms.