The file system for the Moore School Problem Solving Facility of the University of Pennsylvania provides multiple files for various users and allows users to store, retrieve and update information in the files. The working environment when using the file system is on-line, multiple-access and real time. In this kind of environment there exists problems which do not arise in a batch process or in a single-access online environment. One of the problems that arises in using a file system in such an environment is that whenever a user makes changes in a particular file, he may cause other users, who are simultaneously accessing the same file to obtain erroneous information. The objective of this thesis is to provide a solution to the above problem.