A fundamental experimental study is carried out to investigate the unsteady flow organization of an incident shock wave/turbulent boundary layer interaction at Mach 2.1. Planar and tomographic particle image velocimetry (PIV) are used in combination with data processing using the proper orthogonal decomposition (POD), complemented with hot-wire anemometry (HWA) and nonlinear time series analyses. It is found that the global structure of the interaction region varies considerably in time and the mean flow-field is a simplified representation of a more complex instantaneous structure. An inter-relationship appears to exist between the incoming boundary layer, separated flow region, and reflected shock wave. The incoming boundary contains large-scale coherent motions, in the form of three-dimensional streamwise-elongated regions of relatively low- and high-speed fluid. The reflected shock wave region conforms to these regions as they enter the interaction, and may be viewed as a supposition of a streamwise translation and a spanwise rippling. The HWA results reveal that the reflected shock wave region contains energetic frequencies an order of magnitude lower than those found within the undisturbed boundary layer at the same distance from the wall. The time series is shown to be represented as a chaotic attractor in a limited dimensional state-space. This attractor has a rich, underlying structure, which contains the signatures of the low- and high-speed regions as they enter the interaction.