The Shack-Hartmann wavefront sensor (SHWFS) has seen an increasing number of applications over the last 15 years. Originally used for high-energy laser adaptive optics, and then later applied to astronomy, more recently the numerous applications have been developed. The Shack-Hartmann wavefront sensor is used to measure the irradiance and phase distributions of an incident light beam by dissecting the income light with a lenslet array, and then detecting the direction of propagation by analyzing the focal spot pattern (detected with a CCD camera). Since the system is passive (that is, it measures any light that is incident, without the need for a reference beam), it can be used for a variety of situations where interferometry would be difficult. Applications include measurement and characterization of laser beams, optics, and optical systems, metrology of silicon wafers and ultraflat surfaces, as well as extremely rough (ground and machined) surfaces. A SHWFS was used to characterize missile seeker windows in a Mach 7 wind tunnel experiment. In another application, a LWIR sensor was used to measure the next space telescope (JWST) mirror segments during the fabrication process. Currently the most common application is the measurement of the human eye, where instruments based on this technology are used in thousands of clinics around the world. Dr. Neal will describe the basic technology, current implementations, and a number of diverse applications.