The essential results achieved during the ten years since the coming of lasers, in the fields of second, third and higher harmonic generation in reflected and transmitted, continuous and pulsed laser light, frequency mixing processes in various materials, and nonlinear optical properties of liquids, gases, crystals and other materials are reported concisely from the available literature. Stress is laid on the newer and more stimulating experimental results, and on their relation with other nonlinear phenomena.