This monograph offers an alternative explanation for the plasticity of solids. According to it the plasticity or brittleness of solids is determined by the spatial distribution of valence electrons—whether isotropic or anisotropic. In the case of an isotropic distribution, a spherically symmetric interatomic potential forms, leading to additional atomic rapprochement, lattice compression, and ultimately the transformation of the crystal lattice into an unstable state with respect to small shear deformations.The effect of defects on the plasticity of solids is secondary.The findings establish a direct relationship between lattice stability and the plasticity and strength of solids: solids with a stable lattice are strong but exhibit low plasticity, whereas those with an unstable lattice are more plastic but less strong. Furthermore, lattice compression induces both electrical conductivity and plasticity in solids, transforming them into metals.Building on the concept of lattice instability, the monograph introduces a novel model of a plastic crystal. In this model, internal atoms occupy metastable positions, stabilized by a thin surface layer, rendering the crystal in a state of unstable equilibrium. According to this model, plastic deformation occurs as atoms shift toward positions of lower potential energy under external forces.This work will be valuable to specialists in solid-state physics, metallurgy, and materials science. It will also serve as a key resource for researchers, students, and postgraduate scholars in higher education institutions.