The curvature of a nearly plane wavefront can be detected in a shearing interferometer, in which one portion of the amplitude of a wavefront is made to interfere with another portion that has been shifted transversely, or sheared. When used with laser light having a high degree of spatial and temporal coherence, a shearing interferometer may consist of simply an optical-flat held at an angle to the illuminating wavefront so that reflections from the front and back surfaces interfere, as in Fig. 1. This provides an extremely simple tool for checking the adjustment and quality of laser beam expanders. The technique is of sufficient practical value to warrant a description of the simple theory and application.