This book is the first textbook dedicated to the title subject. The objectives of this book are two-fold: To provide insight and understanding of two-phase flow systems or conducting research in this area. This book is aimed primarily at graduate engineering students and industry engineers who are studying two-phase flow in pipes and are interested in improving flow predictions. The CD included with the book contains two programs: FLOPATN, used to predict two-phase patterns that can occur in pipelines of all inclinations, and DPDL, used to predict liquid holdup and pressure drop in a gas-liquid pipeline, incorporating mechanistic models and the most commonly used empirical correlations.