Non-linear optics has become rapidly rising field in physics and materials science. The fast development of this branch of optics has started in 1960 with design of first laser.
Actual interest in materials for second harmonic generation is mainly motivated by rapidly growing area of their technical applications. In this article we have reviewed basics physical principles concerning non-linear optics, categories and particular examples of materials for second harmonic generation, and finally methods used for characterization of non-linear optical properties.