There is an impressive interest in titanium dioxide for its several technological applications which include, e.g., photocatalysis and realization of solar cells. Many of such applications utilize the anatase phase rather than the rutile one, as this appears to be more catalytically active and exhibits a higher electron mobility. In the present study, we address the origin of such differences between the anatase and rutile properties and show that they can be related to a remarkable difference in nature between the electronic levels induced by bulk oxygen vacancies (VOx) in the two TiO2 polymorphs. These levels have indeed different locations in the energy gap and give rise to different localizations of the electronic charge in real space, thus indicating the VOx’s as important candidates to elucidate the different properties of the two TiO2 phases. Present results, achieved by applying a U correction to local spin-density (LSD)–generalized gradient approximation (GGA) methods, stress also the importance of beyond-LSD methods when investigating metal oxides. They show indeed that only such methods permit to reveal the different nature of the VOx electronic states in the two TiO2 phases.