Results of an experimental heat-transfer investigation in regions of separated flow are presented and compared with the theoretical analysis of NACA T N 3792. The average heat transfer for both laminar and turbulent separated boundary layers was found to be from 35 to 50 per cent less than that for equivalent attached boundary layers. The overall scope of the measurements included Mach Numbers from 0.3 to 4.0 and Reynolds Numbers from 1C to 4 X 10. The results for laminar boundary layers agree well with the analysis of T N 3792. The results for turbulent boundary layers, however, disagree considerably. Results of velocity and temperature surveys in the separated turbulent boundary layer are presented and partially explain the discrepancy between the experiments and analysis. The maximum local heat-transfer rates were found to occur in the reattachment region of the separated boundary layers investigated. The effect of transition on heat transfer in the separated laminar boundary layers is described and data showing effects of Mach Number and wall temperature on the transition Reynolds Number of separated laminar flows are also included.