The basics of applied nonlinear echo - generation of the second optical harmonics and parametric light generation. Issues of nonlinear polarized dielectric and phase synchronicity are considered; conclusions and solutions of shortened equations for the generation of the second harmonic in different approximations - in the absence and presence of a wave disorder , in the proximity of the flat waves , limited beams and pulses , in the approximations of the given field and the given intensity of laser radiation , in geometric-optical approximation for divergent beams, including given the heterogeneity of the crystal, Thermal Self-Impacts , non-stationary effects and others. The processes of intraresonator generation of the second harmonic and parametric light generation, as well as features of frequency conversion in biaxial crystals and in crystals with regular domain structure. are considered. A brief history of nonlinear optics. The Appendix provides tables of parameters of the most slender nonlinear crystals and recommendations for their use.