Propelled by advances in software design and increasing connectivity, distributed computational systems are acquiring characteristics reminiscent of social and biological organizations. This volume is a collection of articles dealing with the nature, design and implementation of these open computational systems. Although varied in their approach and methodology, the articles are related by the goal of understanding and building computational ecologies. They are grouped in three major sections. The first deals with general issues underlying open systems, studies of computational ecologies, and their similarities with social organizations. The second part deals with actual implementations of distributed computation, and the third discusses the overriding problem of designing suitable languages for open systems. All the articles are highly interdisciplinary, emphasizing the application of ecological ideas, game theory, market mechanisms, and evolutionary biology in the study of open systems.