The bulk of this chapter investigates introduction of intrinsic defects into crystalline TiO₂ via various synthesis and treatment strategies and their influences on its structural, optical, electronic, and electrical characteristics as well as photoactivity. Wide range of information available in literature is systematically analyzed and critically examined in order to provide thorough understanding regarding the physics and chemistry of this compound and how to control both of them. Special attention is given to reaching clear vision toward implementation of defective TiO₂ as perspective candidate for visible-light-induced photocatalysis given that it enables to show extended absorption characteristics. Finally, several words are dedicated to investigating amorphous TiO₂ as its disordered composition filled with certain concentration of coordination and structural defects originates several unique and very useful features. Thus, once combined with its crystalline counterpart in a correct and proper way, the resulting composition can demonstrate enhanced light-induced performance.