The term “photocatalysis” has recently gained high popularity, and various products using photocatalytic functions have been commercialized. Of all the materials that may be used as photocatalysts, titanium dioxide (TiO2) is virtually the only one that is now and most likely will remain appropriate for industrial application. Water and air purification systems, sterilization, hydrogen evolution, self-cleaning surfaces, and photoelectrochemical conversion are just a few of the products and applications in the environmental and energy domains that make extensive use of TiO2 photocatalysis. This is due to the fact that TiO2 has the lowest cost, most stability, and most effective photoactivity. Furthermore, history attests to its safety for both people and the environment because it has been used as a white pigment since antiquity. This review discusses some important aspects and issues concerning different synthesis methods and their influence on the structure and properties of TiO2, as well as the concept of photocatalysis based on it as a promising biocompatible functional material that has been widely used in recent years. The advantages of TiO2 applications in various fields of science and technology are discussed, including environmental protection, photocatalysis including self-cleaning surfaces, water and air purification systems, hydrogen liberation, photovoltaic energy, cancer diagnosis and therapy, coatings and dental products, etc. Information on the structure and properties of TiO2 phases is presented, as well as modern methods of synthesizing functional materials based on it. A detailed review of the basic principles of TiO2 photocatalysis is then given, with a brief introduction to the modern concept of TiO2 photocatalysis. Recent advances in the fundamental understanding of TiO2 photocatalysis at the atomic-molecular level are highlighted, and advances in TiO2 photocatalysis from the perspective of design and engineering of new materials are discussed. The challenges and prospects of TiO2 photocatalysis are briefly discussed.