This volume contains the proceedings of the Ninth GAMM-Conference on Numerical Methods in Fluid Mechanics, held at the Ecole Polytechnique Federale de Lausanne, Switzerland, on September 25-27, 1991. This conference, as weil as the preceding eight ones, was organized by the GAMM Committee on Numerical Methods in Fluid Dynamics. 1t was probably also the last one in this successful series of conferences, since in the future, a bi-annual European meeting on Computational Fluid Dynamics will be organized by ECCOMAS, a new organization, representing all existing European professional societies...