Presenting studies of human cognition in situations that involve co-operation, expecially situations involving human-computer interaction, this volume aims to find a common thread. The concept sought is one that underlies co-operative behaviour and that is apparent in studies of human cognition, analyses of co-operative systems, and designs of distributed computing systems. The editors take the position that co-operation is a special form of goal directed behaviour among several agents. This position is set forth throughout the book in more formal analyses and examples.