Potassium Dihydrogen Phosphate (KDP) and Deuterated Potassium Dihydrogen Phosphate (DKDP) are nonlinear optical materials (NLO)known for their large NLO coefficients, wide transparency range and high optical damage threshold. These materials are widely used as frequency multipliers for generation of the second, third and fourth harmonics for Nd:YAG Nd:YLF lasers. Both crystals are also applied in electro-optics: as Q-switches for Nd:YAG, Nd:YLF, Ti:sapphire and Alexandrite lasers and Pockels cells. Owing to a possibility of growing large crystals, high laser damage threshold, high optical homogeneity KDP and DKDP crystals are the only materials suitable for making frequency multipliers, Q-switches and Pockels cells, laser damage threshold of which is over 40 J/cm2 (at lambda=1.06 µm, tau=3 ns). This allows to use them in the devices for laser fusion. KDP and DKDP crystals possess electro-optical, nonlinear and piesoelectric properties, high laser damage threshold and are used as the base for the devices and systems of laser engineering: frequency multipliers, modulators, gates, deflectors.