Overlay performance is usually determined by exposing wafers twice, processing the wafer, and then using vernier or electrical probe techniques to measure the overlay error. In this paper, we describe a very different technique where the overlay error is evaluated in situ on the lithographic exposure tool. The basis of this approach is the examination of moire fringes between the projected image of a grating test mask and a grating test wafer. This is a particularly attractive approach to lithographic metrology because the moire fringes contain information about both resolution and overlay error. A measurement instrument has been constructed which attaches to a MicralignTM projection printer. The fringe intensity is measured by a solid state detector array positioned on an image of the illuminated arc-shaped field. Since the overlay measurement can be done many times per second, we can directly observe vibrations between mask and wafer.