This book is based on the author's PhD thesis which was selected during the 1993 ACM Doctoral Dissertation Competition as one of the three best submissions.The focus of this work is on the issue of availability in distributed file systems. It presents the important new technique called disconnected operation, in which clients mask failures and voluntary network detachments by emulating the functionality of servers where actual server-oriented solutions are inadequate. This permits client operation even under complete isolation from the server; the clean integration of mobile computers into the system is an important side-effect of the new technique. The design and implementation of disconnected file service in a working system, the Coda file system, is described in detail.