This is the third of three volumes containing the proceedings of the Intemational Colloquium 'Free Boundary problems: Theory and Applications', held in Montreal (Canada) from June 13 to June 22, 1990. The main part of this volume studies the flow of fluids, an area which has led to many of the classical free boundary problems. The first two sections contain the papers on various problems in fluid mechanics. The types of problems vary from the collision of two jets to the growth of a sand wave. In the next two sections porous flow is considered. This has important practical applications in fields such as petroleum engineering and groundwater pollution. Some new and interesting free boundary problems in geology and engineering are treated in the final section. Readership: Researchers and postgraduate students with interests in free boundiry problems, mathematical modeling, partial differential equattions, functional analysis, numerical analysis and asymptotic analysis. lt may also be of interest to engineers, geolouists, materials scientists, physicists. chemists and biochemists. The aim of this series is to disseminate important new material of a specialist nature in economic form. It ranges over the whole spectrum of i-nathematics and also reflects the changing momentum of dialogue between hitherto distinct areas of pure and applied parts of the discipline.