—Since flow separation occurs readily from a highly swept leading edge, but gives rise in general to a steady flow, it is proposed that the rational approach to the aerodynamic design of slender wings is to attempt to control, rather than to suppress, these separations. This leads to the suggestion that the leading edges should be sharp, and that the wing should be shaped so as to make them attachment lines at one attitude (the design attitude) at which classical wing theory can be applied. It is argued, further, that if the leading edge separations are to develop regularly with change of attitude of the wing, separation must occur only from the trailing edge at the design attitude; and the velocity field favourable to boundary layer development without separation forward of the trailing edge is discussed. Subject to the restrictions thus imposed on the design, low drag is sought at the design attitude. This leads to the consideration of a particular class of doubly curved mean surfaces satisfying the leading edge condition, onto which thickness distributions are superposed so as to provide favourable velocity fields together with low drag. A number of examples are considered, using slender thin wing theory for flexibility, to illustrate the manner in which plan form and thickness distributions affect the pressure distribution, and to indicate the relatively high lift/drag ratios which seem feasible. Some consideration is given to the limitations of the theory used and to the further developments which seem desirable.