Abstract : The prediction of the laminar-turbulent transition in the boundary layer on a hypersonic vehicle forebody is important tooptimize the air inlet of the associated scramjet engine, but is still very difficult after half a century of intensive research onthe subject. In this work, numerical and experimental approaches are applied and compared. Experimentally, the naturaltransition is detected at Mach 4 and Mach 6 in the blow down wind tunnel T-313 in ITAM Novosibirsk using Pitot pressuremeasurements. In the impulse AT-303 wind tunnel in ITAM, the natural transition at Mach 6 and the roughness inducedtransition at Mach 8 are detected using an optical method based on thermosensitive paints. These tests have beenperformed on a 1/3 scale model. All the trips tested have shown their effectiveness. The theoretical prediction of thenatural transition has been performed using the local modal linear stability theory coupled with the eN method. In flight, onthe full scale forebody, N factors hardly reach 8 to 9, which is insufficient for the transition. To apply the method to groundtests, the wind tunnels transition N factors are needed. They are obtained from calibration tests on a flat plate in T-313. Avery good agreement with experiments is found at Mach 4. At Mach 6, the presence of inflexional crossflow instabilitynear the nose of the body must be taken into account, which gives also a good agreement. Stability calculations havebeen done for mean flow solutions obtained by numerical simulations (CFD) of flight or ground tests conditions. Thesesimulations have also helped to understand the structure of the flow around the forebody and to design efficiently theexperimental setup.