Optical Parametric Generation (OPG) is an inverse process of sum Frequency Generation. It splits one high-frequency photon (pumping wavelength ,λp) into two low-frequency photons(signal,λs ,and idler wavelength, λi).If two mirrors are added to from a cavity, an Optical Parametric Oscillator (OPO) is established. For a fixed pump wavelength, an infinite number of signal and idle wavelengths can be generated by tilting a crystal. OPO is an excellent source for generating wide tunable range coherent radiation. BBO,KTP,LBO and LiNbO3 are good crystals for OPO and Optical Parametric Amplifier(OPA) applications.