The paper discusses the topics of ‘two dimensional’ shock‐induced separation, glancing interaction and hypersonic viscous interaction. Wherever possible both laminar and turbulent boundary layers are considered and reference is made to experimental data and to the results of mathematical modelling.Many quasi‐two dimensional tests have been made to study the flow past a compression corner. The data are reviewed with particular reference to incipient separation and the effects of separation on the pressure and heat transder distributions are described.One of the most important three‐dimensional interactions concerns the way in which a shock wave generated by a fin, pylon, tail‐plane or wing influences the boundary layer growing over the surface to which the shock‐generator is attached. A number of results are examined for a fin mounted on a flat plate or side wall. The sparate and combined effects of incidence, sweep and blutness are described.Even when no separatin occurs the boundary layers at hpersonic speeds can be be thick enough to significantly modify the pressure distribution around a given geometric shape. The example considered here is laminar viscous interaction over a trailing edge control surface and a simple analytical method is used to demonstrate the reductions in control effectiveness.The paper ends with some thoughts on future research topics and the facilities needed to pursue them.