A review and analysis of studies concerning the nature of Poisson's ratio μ of glassy systems are presented. The value of μ is a more pronounced structure-sensitive property than are elasticity moduli. The unique relation of μ with the Gruneisen parameter is substantiated. In this connection, the interrelation of harmonic (linear) and anharmonic (nonlinear) characteristics is separately considered. Poisson's ratio proves to be a single-valued function of parameters characterizing dynamic properties and critical processes and is sensitive to the lattice dynamics and the atomic-molecular structure of glasses. The structural features of isotropic solids with the negative Poisson's ratio (coefficient of transverse deformation) are discussed.