In the present study, the effect of various twodimensional surface defects (forward-facing steps and ramps, backward-facing steps and ramps, gaps and steps-and-gaps) on boundary-layer transition was experimentally investigated in the transonic regime. A laminar profile was specifically designed and manufactured by ONERA so as to allow for a maximum of defects to be tested simultaneously, and to include resin pockets to accurately monitor laminar-turbulent transition using infra-red thermography. Transition was also characterised using the âN model based on linear stability calculations. Relatively good agreement with existing âN models for forward and backward facing steps as well as gaps was found, indicating that these models, which were mostly developed for incompressible flows, can still be used as a conservative estimate for compressible flows. More practical defects referred as step-and-gap were also considered.