Cooling problems have recently been occupying a considerable portion of the time of the Aerodynamics Department at the Royal Aircraft Establishment. The work had hitherto been of a theoretical nature combined with tests in the small tunnels. The new 24ft. tunnel has provided us with a very valuable piece of apparatus to deal with the problem. The work is proceeding and is still incomplete, but we are beginning to have a clear idea of the principles involved. In this lecture I propose to describe briefly the tunnel and its working and the work we have been doing in it. The lecture is given by permission of the Director of Scientific Research, Air Ministry, but the views expressed represent only my own opinions.