Over the last few years, object-oriented programming has been recognized as the best way currently available of structuring software systems. It emphasizes grouping together data and the operations performed on them, encapsulating the whole behind a clean interface, and organizing the resulting entities in a hierarchy based on specialization in functionality. In this way it provides excellent support for the construction of large systems. Up to now, there has been relatively little effort to develop formal theories of object-oriented programming. However, for the field to mature, a more formal understanding of the basic concepts of object-oriented programming is necessary. This volume presents the proceedings of the School/Workshop on Foundations of Object-Oriented Programming (FOOL) held in Noordwijkerhout, The Netherlands, May 28 - June 1, 1990. The workshop was an activity of the project REX (Research and Education in Concurrent Systems).