In this short paper on the transonic jet-driven tunnel, I will place the emphasis on the word transonic and deal only with the jet drive as it affects the design or operation of the transonic working section. I also would like to say that, although a fair amount of the information discussed afterwards will refer to the D.H. transonic tunnel, the problems are more or less general to the jet–driven tunnel and some will even be applicable to electrically–driven transonic tunnels using diffuser suction.