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.