Band gap engineering of TiO2 has attracted many researchers looking to extend its applicability as a functional material. Although TiO2 has been commercialised in applications that utilise its special properties, its band gap should be modified to improve its performance, especially as an active photo catalyst. Reduction of TiO2 under a hydrogen atmosphere is a promising method which can increase the visible-light absorption efficiency of TiO2 and enhance its electrochemical and other properties related to electronic band structure. In this second review paper, the production and influence of O vacancies (VO) and other defects, such as interstitial cations, under vacuum and hydrogen are reviewed for the common phases of TiO2. The particular modification TiO2–x in which O is randomly removed from the crystal structure is considered in detail. Despite early evidence that hydrogen is absorbed into the bulk of TiO2, the action of hydrogen has become controversial in recent years, with claims that surface disorder is responsible for the enhanced photoactivity induced by exposure to hydrogen. The many published experimental and density-functional-theory modelling studies are surveyed with the aims of determining what is agreed or contested, and relating defect structure to band structure. It is concluded that further work is needed to clarify the mechanisms of defect production and defect diffusion, as well as the origins of the numerous sample colours observed following treatment in vacuum or hydrogen.