Collaborative virtual environments are multi-user virtual realities which actively support communication and co-operation. This book addresses the theory, design, realisation and evaluation of such systems, with a particular emphasis on support for large numbers of distributed users. A broad approach is taken, which ranges from the sociology of interpersonal communication to the management of communication in distributed systems. The emphasis on multi-user environments distinguishes this book from the many general books on virtual reality which only deal with single-user systems. This book presents: models of multi-party awareness and interaction in space-based systems; detailed designs of two prototypes (MASSIVE-1 and MASSIVE-2); experiences with collaborative virtual environments created using these; and analyses of the corresponding network requirements. Many of these results and ideas are applicable to other systems and approaches.