Many inorganic crystals show strong nonlinear optical properties, and there still exists a need for new materials. One of them is barium borate existing in two modifications i.e. a high temperature phase α-BaB2O4 and low temperature phase β-BaB2O4. The β-phase in single crystal form (BBO) belongs to trigonal structure and possess high nonlinearity. The a-phase crystals are centrosymetric and for nonlinear applications are not useful. Attractive features of BBO crystals are: large nonlinear optical coefficients about six times higher as compared with KDP, large birefringence, transmision range between 190 and 3500 nm, damage threshold as high as 10 GW/cm2 measured with 100 ps pulses, low absorption, moisture insensitivity, and good mechanical and thermal properties. BBO crystals are used successfully for efficient generation of ultraviolet light from visible light source. They are suitable for use with the most powerful lasers for second and higher harmonic generation laser light and as an optical parametric amplifier (OPA) and optical parametric oscillator (OPO). Crystal structure of BBO has similarities with a molecular crystal structure and origin of its nonlinear properties is reminiscent of organic nonlinearity of organic crystals. The structure is built from B3O6 rings packed up perpendicularly along the c-direction of hexagonal unit cell.