THE WHOLE GAMUT OF SECURITY PROBLEMS IS CONSIDERED, INCLUDING PHYSICAL ATTACKS, MORE SUBTLE SOFTWARE PENETRATION, AND ACCIDENTAL THREATS SUCH AS MAGNETISM AND HARDWARE ERROR. BECAUSE COMPUTER SYSTEMS HAVE ONLY RARELY BEEN DESIGNED WITH SECURITY IN MIND, ABSOLUTE INVIOLABILITY IS GENERALLY CONSIDERED TO BE IMPOSSIBLE. A CRITERION FOR THE PROTECTION STRUCTURES CONSIDERED IS THAT THEY MAKE THE COST OF PENETRATING A SYSTEM SO HIGH THAT THIS COST EXCEEDS ANY POSSIBLE GAIN THE ILL-GOTTEN INFORMATION COULD PROVIDE. VIGOROUS ATTENTION IS GIVEN TO DEFENSES AGAINST UNAUTHORIZED ACCESS: ADMINISTRATIVE AND HARDWARE SAFEGUARDS, REMOTE TERMINAL SECURITY, PROTECTION OF THE OPERATING SYSTEM DESIGN, DATA TRANSMISSION, AND CRYPTOGRAPHY. A SURVEY OF A NUMBER OF IMPLEMENTED PROTECTION SYSTEMS - THE UNIX, THE IBM RUSH AND TSS/360, THE HONEYWELL MULTICS, THE HONEYWELL 6000 SERIES, AND MANY OTHERS - ILLUSTRATES WHAT IS AVAILABLE AND PROVIDES LEADS FOR ADDITIONAL INFORMATION SOURCES. A 17-PAGE BIBLIOGRAPHY AND AN INDEX ARE PROVIDED. (AUTHOR ABSTRACT)...ELW