Flow field studies of the shock wave and boundary layer development on a sharp flat plate are presented for a region of rarefied flow that bridges the gap between a classical hypersonic boundary layer downstream and a kinetic flow model at the leading edge. The measurements give a comprehensive picture of the flow pattern in the 'merged layer' or 'viscous layer' regime, which exists upstream of the region of validity of hypersonic viscous interaction theory. The results are derived from a combination of several probing and optical techniques and surface pressure measurements. From the detailed measurements, a true scale physical model of the flow field is constructed for the merged layer regime. One of the main features of the model is a thick, curved shock wave which decreases in strength as the leading edge is approached, even though the shock angle increases. Density profiles across the shock and viscous layers show that the structure of the flow field is quite different from the classical picture of a hypersonic boundary layer beneath an oblique Rankine-Hugonio shock wave. (Author).