This book continues the tradition of the "Multiphase Science and Technology" series by presenting material on multiphase systems which has a high level of review and quality assurance. The content of this particular volume had its origins in the International Workshop on Two Phase Flow Fundamentals, Gaithersburg, Maryland, September 1985. This material represents an important landmark in two-phase flow studies since it contains an archival reference source on closure relationships in two-phase flow modeling on experimental data sets which are suitable for such modeling, and on numerical bench marks against which modeling codes can be tested for the robustness of the numerical schemes.