A complete theory of Maker fringes in nonabsorbing isotropic and uniaxial crystals has been derived which includes all the corrections necessary for making precise determinations of nonlinear optical co‐efficients. These corrections include finite beamwidth effects and multiple reflection corrections. Comparison of this theory with extensive experimental data on the Maker fringes in quartz, ADP, and KDP shows agreement to within the experimental accuracy of about 5&percent; on the Maker fringe envelopes and to better than 1&percent; on the coherence lengths. We conclude from this study that a careful analysis of Maker fringes can yield precise values of the nonlinear optical coefficients and coherence lengths in isotropic and uniaxial crystals. This is of great importance in establishingaccurateandreliablestandards in the field of nonlinear optics.