The origin of shock unsteadiness in a Mach 2.9 turbulent reattaching shear layer was investigated experimentally using temporally resolved flow visualization and measurements of wall pressure fluctuations. In this flow, the separation point of a turbulent boundary layer is essentially fixed at a backward-facing step, and the reattachment point is free to move along a ramp. In order to examine the influence of disturbances originating in the incoming shear layer, artificial disturbances were introduced into the flow through steady air injection in the vicinity of separation. The effect on the reattachment shock system was dramatic: the intensity of the pressure fluctuations and the amplitude of the shock motion increased substantially, and power spectra of the pressure fluctuations showed a distinct shift to lower frequency. The spectra collapsed onto a common curve in non-dimensional coordinates based on a length scale derived from two-point cross-correlations of the flow visualization data and a convection velocity derived from cross-correlations of the pressure measurements. The data were compared to a theory developed by Plotkin (1975), which is based on perturbation of a shock by random fluctuations in the incoming turbulent flow. Plotkin's model mimics the manner in which relatively broad-band perturbations in the incoming turbulent flow lead to relatively low-frequency motion of the separation bubble and its associated shock system. It is an excellent fit to separation shock motion, such as that generated in a blunt fin flow (briefly illustrated here). In the present shear layer flow, this low-frequency motion was detectable in the spectra near reattachment, but contained considerably less energy relative to the shock motions caused by direct perturbations by the incoming turbulent structures. These results indicate that the shock motion in the reattaching shear layer is primarily caused by organized structures in the incoming turbulent flow.