The interaction between an underexpanded normal jet and a hypersonic cross flow has been studied experimentally. Although the main context of the work is reaction control, many fundamental aspects of the interaction flow field have been included. Experiments were performed in the Southampton University light piston, isentropic compression tube (SULPICT) at a free stream Mach number of 6.7. Two flat plate models were tested, the upper surface of the plate being aligned with the tunnel centre-line. One model was used for slot injection and the other for axisymmetric injection. Free stream conditions were held constant while the jet stagnation pressure, injectant gas type, sonic nozzle area and, for axisymmetric injection, nozzle exit Mach number were varied. Test conditions were such that the boundary layer ahead of the jet interaction (J.I.) region was laminar. Schieren photography, surface temperature and oil flow visualisations were used to examine the interaction flow field. The J.I. force was evaluated from measurements of the surface pressure distribution. For slot injection, side plates were installed during some tests to restrict lateral venting from the recirculation zone. Slot and axisymmetric free jets were also investigated. From the slot jet study, it was found that the main characteristics of the upstream J.I. region are due to the (size of) obstacle that the jet presents to the oncoming flow. A normalised J.I. parameter was introduced, related to the size of the jet obstacle, which correlated the current J.I. data. The parameter included an effective back pressure term based on the local flow conditions. Correlation of slot J.I. data was affected by significant lateral venting from the separated flow region. The influence of injectant gas type as found to be most negligible on the interaction force, although Helium injection did produce a relative increase in the surface pressure distribution. Heat transfer and oil flow visualisations confirmed the presence of a counter-rotating vortex pair ahead of the jet and revealed a complex three-dimensional flow downstream of the jet exit.