This chapter is a discussion of one of the most powerful and useful parts of the Tru64 UNIX system the Advanced File System (AdvFS). If you are familiar only with the UNIX file system (UFS) from either Tru64 UNIX or another vendor's UNIX, then you will need to learn some concepts to fully appreciate AdvFS. The first question that is often asked concerning the AdvFS by those unfamiliar with it is: "UFS has been around for so many years and continues to work well, so why do I need AdvFS?" This question is answered simply that AdvFS offers powerful features that are not and will never be available in UFS. The authors believe that AdvFS is essential for those users with large file systems (and they get bigger every day) and 24- hour, year-round uptime requirements. UFS does not provide the features necessary for managing very large file systems. In addition, AdvFS was designed from the beginning, like Tru64 UNIX, to be a 64-bit file system. Thus, large file systems have always been supported, and the theoretical limits of AdvFS will not be reached for quite some time.