The proposed chapter will be based on the original work of the authors on the photoelectrochemical properties of undoped titanium dioxide and modified TiO₂ both in a form of thin films and bulk ceramics. Described modifications will include: TiO₂:Cr, TiO₂:Nb, TiO₂:Sn, TiO₂:VO₂, TiO₂:WO₃, TiO₂:Au (Ag). The following issues will be addressed: (1) the photoelectrolysis of water as a source of hydrogen, (2) photoactivity of TiO₂ (3) experimental methods of study of the photo-active materials such as photoelectrochemical determination of the flat band potential, impedance spectroscopy, uv and vis spectroscopy with an integrating sphere (4) optical properties of undoped titanium dioxide and modified TiO₂ (5) photo-electrochemical properties of undoped titanium dioxide and modified TiO₂. The influence of the material modification on the electronic, optical and photoelectrochemical properties will be described. In particular I-V and C-V characteristics, the photo- responses, relaxation times, flat band potential, band gap, etc. will be discussed in detail.