Describes the phenomena fundamental to efficient optical frequency mixing of laser beams in transparent, acentric birefringent crystals, with particular emphasis on phase-matching, effective nonlinear coefficients and results for focused beams. Methods for measuring nonlinear coefficients, for predicting them by semi-empirical rules and for seeking improved crystals are reviewed. The properties of crystals known at this time to be useful for efficient phase-matched optical frequency mixing are tabulated; features important for the growth and preparation of samples with high optical quality are discussed. Recent achievements in applying the crystals are reviewed in sections on optical second-harmonic generation of laser light, optical parametric oscillators as tunable laser-like sources, infrared signal and image upconverters and difference-frequency generators. Applications to other technologies (piezoelectricity, pyroelectricity, electro-optic modulation) are briefly mentioned. Prospects of developing improved crystals or of using existing crystals in more sophisticated ways are assessed.