This is the second volume of Multiphase Science and Techno Jogy, a new international series of books intended to provide authoritative overviews of im- portant areas in multiphase systems. The alm is to have systematic and tutorial presentations of the state of knowledge in various areas. The objective of the chapters is to allow the nonspecialist reader to gain an up-to-date idea of the present state of development in a given subject. The response to Volume 1 of the se ries has been very positive, and we believe that the present volume will be equally weil received. Volume 1 was concerned entirely with gas-liquid systems, and the first four chapters of the present volume also relate to such systems. However, the inten- tion of the se ries is to cover a wide range of multiphase systems, and we are, therefore, pleased to include in the present volume chapters that refer to liquid- liquid and gas-solid multiphase flows, respectively. The first chapter in the present volume is by Professor A. E. Dukler of the University of Houston, Texas, and Professor Y. Taitel of Tel-Aviv University, Israel.