Over the past two decades, two-phase flow and heat transfer problems associated with two-phase phenomena have been a challenge to many investigators. Two-phase flow applications are found in a wide range of engineering systems, such as nuclear and conventional power plants, evaporators of refrigeration systems and a wide vari- ety of evaporative and condensive heat exchangers in the chemical industry. This publication is based on the invited lectures presented at the NATO Advanced Research Workshop on the Advances in Two-Phase Flow and Heat Transfer. The Horkshop was attended by more than 50 leading scientists and practicing engineers who work actively on two-phase flow and heat transfer research and applications in dif- ferent sectors (academia, government, industry) of member countries of NATO. Some scientific leaders and experts on the subject matter from the non-NATO countries were also invited. They convened to discuss the state-of-the-art in two-phase flow and heat transfer and formulated recommendations for future research directions. To achieve these goals, invited key papers and a limited number of contributions were presented and discussed. The specific aspects of the subject were treated in depth in the panel sessions, and the unresolved problems identified. Suitable as a practical reference, these volumes incorporate a systematic approach to two-phase flow analysis.