In the past few years the term “barrier” has become popular in aeronautics. Not so long ago we heard of the sonic barrier which in the minds of many laymen represented a solid wall of compressed air which would not let an airplane through at the speed of sound. More recently the expression “thermal barrier” has been coined to indicate that the heating effect of air will set a definite limit to the speed of aircraft that can never be surpassed. As far as the sonic barrier is concerned, events have shown that supersonic flight is well possible. As a matter of fact, bullets flew at supersonic speeds even during the last century, and if no law of nature prevented them from doing so, it was unreasonable to expect airplanes to be stopped by the sonic barrier. The author is convinced that in a similar manner the difficulties presented by the high temperatures accompanying flight at supersonic speeds will be overcome through research and development. Thus the thermal barrier is not a stone wall that will shatter all hopes of high supersonic flight.