This book is the standard text book for elastoplasticity which is explained comprehensively covering the rate-independent to rate-dependent deformations and the infinitesimal to finite deformations for metals, soils, polymers, crystal plasticity, etc. and the friction phenomenon. Concise explanations on vector-tensor analysis and continuum mechanics are provided first as a foundation for elastoplasticity theory, covering the underlying physical concepts, e.g. convective time-derivatives, pull-back and push-forward operations, objectivity, work-conjugacy and multiplicative decomposition of deformation gradient tensor. Then, the following theories/models are described in detail, almost of which have been proposed by the author and thus they are explained comprehensively, which can never be attained by the other plasticity models, e.g. the Chaboche-Ohno and the Dafalias-Yoshida models which assume the purely-elastic domain.1) The subloading surface concept underling the cyclic plasticity is introduced, which insists that the plastic deformation develops as the stress approaches the yield surface. Thus, the smooth elastic-plastic transition leading to the continuous variation of the tangent stiffness modulus is described. 2) The subloading-overstress model is formulated by which the elastoplastic deformation during the quasi-static loading and the viscoplastic deformation during the dynamic and further impact loading can be described by the unified equation. 3) The hyperelastic-based (visco)plasticity based on the multiplicative decomposition of deformation gradient tensor is formulated for the exact descriptions of the finite elastic and (visco)plastic deformations.4) The subloading-friction model is formulated for the exact description of the dry and the fluid (lubricated) frictions at the general rate of sliding from the static to the impact sliding. Thus, all the elastic and inelastic deformation/sliding phenomena of solids can be described accurately in the unified manner by the subloading surface model which will be engraved as the governing law of irreversible deformation of solids in the history of solid mechanics.