Potassium Dihydrogen Phosphate (KDP) and Potassium Dideuterium Phosphate (KD*P) crystals are among the most widely-used commercial NLO materials. They feature good UV transmission, high damage threshold and high birefringence, though their NLO coefficients are relatively low. They are commonly used for doubling, tripling and quadrupling of Nd:YAG lasers at room temperature. In addition, they are also excellent electro-optic crystals with high electro-optic coefficients, widely used as electro-optical modulators, Q-switches, and Pockels Cells, etc.