An overview of the basic concepts, methods and applications of nonlinear low-dimensional solid state physics based on the Frenkel-Kontorova model and its generalizations. The book covers many important topics such as the nonlinear dynamics of discrete systems, the dynamics of solitons and their interaction, commensurate and incommensurate systems, statistical mechanics of nonlinear systems, and nonequilibrium dynamics of interacting many-body systems.