The problem of unauthorized access and use (to steal or alter data, software, or hardware) is discussed; and four main areas of abuse are delineated: manipulation, spying, sabotage, and theft of online time. Strategies for prevention, structural considerations, personnel and organizational procedures, and EDP checks are reviewed. Possible prevention strategies are considered, including method of construction of the processor, the structure of the operating system, checks on the data base, and user supervision. The VMS system is analyzed, and it is concluded that systems using VAX processors fulfill all the criteria for operation within mid-level security requirements. This conclusion is substantiated by U.S. Department of Defense studies. Figures, tables, and 95 notes and references.