Progress in Aeronautical Sciences, Volume 5, contains six articles that belong to the field of aerodynamics. This accords well with one of the declared purposes of this series, to serve the general reader with accounts of those parts of the subject remote from his specialized interest, not only because his field of specialization is likely to change during his career but also because we are all students of one science. The first paper gives a comprehensive survey of low-speed wind-tunnels, those early tools of the aerodynamicist which seem likely to retain their importance indefinitely. The second deals with a basic feature of the separation of a laminar boundary layer from a wall in two-dimensional flow: the bubble. The third reports on the more theoretical aspects of the IUTAM Symposium Transsonicum, which afforded a reunion of almost everyone who at one time or another had contributed to our knowledge of flows of mixed type. The fourth paper is concerned with a particular aspect of the theory of transonic flows and gives an exhaustive survey of this work. The fifth paper reports on a symposium devoted to rarefied gas dynamics. The final paper presents corrections to an article on the theory of sonic bangs, published in Volume 1 of this series.