The optimum coupling for intracavity second harmonic generation has been theoretically studied. It has been shown that, both for a four- and three-level laser, one can get as much power at the second harmonic wavelength as that which can be obtained at the laser wavelength. It is also shown that, for both cases, an optimum value for

exists, where

is the nonlinear crystal length, and

and w
0are the spot sizes of the laser beam, respectively, in the laser material and in the nonlinear crystal.