A wavefront sensor has been designed to measure wavefront aberrations occurring during astronomical observations with high spatial and temporal resolution. Results of laboratory experimentation are presented. The sensor consists of a grating lateral shear heterodyne interferometer using a solid-state detector array. It has high light efficiency and is a self-referencing interferometer, thus allowing the use of white-light extended astronomical sources for the measurements. The wavefront sensor employs a heterodyne technique and is insensitive to intensity variations across the pupil and detector nonuniformities. To obtain a wavefront map, two sets of orthogonal wavefront difference data from the lateral shear interferometer are required. Both sets are measured simultaneously using a single laminar grating to produce the sheared diffraction orders and a single solid-state detector array placed in a plane conjugate to the telescope pupil plane. The signal from the detector array is digitized and fed into a computer where the wavefront differences are calculated. From these differences a wavefront map on a square array of 24 by 24 points is obtained by filtering the data in the spatial frequency domain. The time required for the acquisition of a complete set of data for one wavefront map is 13.3 ms. A wavefront reconstruction may be done within several milliseconds by using fast algorithms and floating point processor hardware.