Viscous-inviscid interaction is an important and difficult problem in transonic aerodynamics. Unfortunately, numerical solutions of the NavierStokes equations are not presently a practical method for routinely solving such problems due to computer resource requirements. Consequently, much research has been done and much is still going on with regard to coupling inviscid and viscous flow solvers for treating viscous—inviscid interaction. Lock [23] and Melnik [20] have reviewed interaction methods. For the most part, these methods consist of using potential flow inviscid solution methods and attached flow viscous solution methods. Inverse boundary-layer methods are being used in some instances (see Le Balleur [308] for a review) in order to include separated flow.