The study of multiphase flows is of utmost interest for engineers who are more or less inevitably faced with them when handling various industrial processes or when dealing with environmental problems such as the dispersion of pollutants. It is also a large kingdom assembling many beautiful and weird landscapes in which wandering researchers may be caught by the fascination of precious stones or mysterious insets to deep and obscure caverns. Unfortunately, it is also an historically disconnected field of research, as testified by any textbook contents or by the scientific programs of conferences devoted to multiphase flows. For instance, is there a relation between fluidization and the study of interfacial waves, or between the behaviour of an annular film of liquid and the one of a free surface heated from below? The answer is indeed: yes. To help reveal some unity behind the avatars of multiphase flow behaviours, it has been decided to focus the interest on the instability phenomena. This book therefore provides the reader with most of the papers which have been accepted and/or presented at the international symposium on "instabilities in multiphase flows" held at the National Institute for Applied Science (INS A) in Rouen, France, from the 11 th of May to the 14th of May 1992. The topic of the conference has produced a strong emphasis on instability theory and nonlinear dynamics, including chaotic phenomena.