The structure and semantics of actor languages -- Designing an object-oriented programming language with behavioural subtyping -- A layered semantics for a parallel object-oriented language -- A proof system for the language POOL -- Object-oriented programming versus abstract data types -- Object-oriented specification in LOTOS and Z, or my cat really is object-oriented! -- A categorial theory of objects as observed processes -- Net-based description of parallel object-based systems, or POTs and POPs -- Describing, structuring and implementing objects -- Modelling features of object-oriented languages in second order functional languages with subtypes -- Graph grammar-based description of object-based systems -- An actor-based metalevel architecture for group-wide reflection -- Producing abstract models for object-oriented languages.

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).