Unsteady flow over shallow cavities (L/D = 4 and L/D = 8) at Mach = 0.8 is investigated experimentally. Data acquisition in and above two-dimensional rectangular cavities is accomplished optically via a multiple exposed Shack Hartman sensor. This technique provides a time-accurate picture of the density behavior in and above the cavity without the need to insert any type of recording device into the flow. The Shack Hartman sensor measures the change in slope of an optical wave front passing through the test area. The change in phase angle may be calculated from this change in slope of the optical wave front The density distribution is then found from the change in phase angle distribution. An in-draft tunnel was designed and built to produce the desired Mach number at a Reynolds number of 11.0 xlO6 per meter. Pressure and temperature in the tunnel, suppfy reservoir and receiving reservoir are measured using traditional pressure and temperature sensors. The temporal behavior of the shear/mixing layer and temporal behavior of the flow structures in the cavity (position and velocity) are analyzed. The impact of the interaction of the shear layer and the cavity structures on the pressure coefficient distribution on the surface of the cavity is also analyzed.