Diffraction and interference processes in Nonlinear and Relaxed Optics are discussed. Nature and basic peculiarities of formation and development these phenomena are discussed. Influence diffraction and interference processes on the formation various processes of Nonlinear Optics (self-focusing, self-trapping, nonequilibrium gratings, holography and other) is analyzed. The role of diffraction on processes of Relaxed Optics (diffraction layering, irreversible gratings and frozen the proper phenomena of Nonlinear Optics in irradiated matter) is discussed. The corresponding experimental data and models are represented too. Perspective of development and use these methods are analyzed too.