The main reasons for the appearance of low-intensity "extra" bands (not corresponding to fundamental lattice vibrations) in the Raman spectrum of nonstoichiometric nonlinear optical LiNbO₃ crystals are discussed in the review. These bands arise due to the formation in the crystal of two-particle states of acoustic vibrations with a total wave vector equal to zero, strong anharmonicity of fundamental vibrations and the presence of microstructures (clusters). The structure of the latter differs from the structure of the crystalline matrix. The presence of microinclusions of impurity phases of other lithium niobates and the effect of photorefraction play a role in the appearance of "extra" bands. It has been shown that "extra" bands of the Raman spectrum can serve as a new highly sensitive tool for studying the structural perfection of LiNbO₃ crystals.