In this work the present state of art about the rare earth and transition metal ion centers phenomenon in LiNbO₃ has been analyzed. Particular emphasis has been given to lanthanide trivalent rare earth ions (Pr³⁺, Nd³⁺, Eu³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺ and Yb³⁺) and to the Cr³⁺ ion, where most of information has been reported. The structure of these centers is consistent with different off-centered positions of the optically active ions inside the Li⁺ octahedral site. Stoichiometry and codoping effects on the center distribution are discussed. Finally, possible models to explain the origin of the different centers are discussed.