Potassium niobate (KNb03) is well suited for efficient frequency doubling of near infrared laser light, due to its large nonlinear optical coefficients, d32 = (18.3 ± 0.3)pm/V and d31 = (15.8 ± 0.3)pm/V at λ = 1064 nm, and the possibility of noncritical 90° Type I phase-matching. Tunable second harmonic generation (λ = 420 to 475 nm) from low power (5 to 100 mW) cw and pulsed AlxGai_xAs and dye lasers has been demonstrated. Higher conversion efficiencies have been achieved by intracavity frequency doubling of a cw dye laser operating at wave-lengths around λ = 860 nm. The second harmonic of a pulsed Styryl-9 dye laser and a mode-locked Nd:YAG laser could be generated with efficiencies as high as 28% and 32% respectively. Efficient 90° phase-matched sum frequency generation of a Nd:YAG signal (λ = 1064 nm) and a ruby pump wave (λ = 694.3 nm) has been demonstrated. New accurate measurements of the non-linear optical coefficients of KNb03 at the wavelengths 1064 nm and 826 nm are presented.