THE FOCUS IS ON TWO SYSTEM STRUCTURING CONCEPTS THAT SUPPORT SECURITY; NAMELY, SMALL PROTECTION DOMAINS AND EXTENDED-TYPE OBJECTS. THESE TWO CONCEPTS ARE ESPECIALLY PROMISING BECAUSE THEY ALSO SUPPORT RELIABLE SOFTWARE BY ENCOURAGING AND ENFORCING HIGHLY MODULAR SOFTWARE STRUCTURES - IN BOTH SYSTEMS SOFTWARE AND IN APPLICATIONS PROGRAMS. SMALL PROTECTION DOMAINS ALLOW EACH SUBUNIT OR MODULE OF A PROGRAM TO BE EXECUTED IN A RESTRICTED ENVIRONMENT THAT CAN PREVENT UNANTICIPATED OR UNDESIRABLE ACTIONS BY THAT MODULE. EXTENDED-TYPE OBJECTS PROVIDE A VEHICLE FOR DATA ABSTRACTION BY ALLOWING OBJECTS OF NEW TYPES TO BE MANIPULATED IN TERMS OF OPERATIONS THAT ARE NATURAL FOR THESE OBJECTS. THIS PROVIDES A WAY TO EXTEND SYSTEM PROTECTION FEATURES SO THAT PROTECTION CAN BE ENFORCED IN TERMS OF APPLICATIONS-ORIENTED OPERATIONS ON OBJECTS. THIS SURVEY ALSO EXPLAINS ONE APPROACH TOWARD IMPLEMENTING THESE CONCEPTS THOROUGHLY AND EFFICIENTLY - AN APPROACH BASED ON THE CONCEPT OF CAPABILITIES INCORPORATED INTO THE ADDRESSING STRUCTURE OF THE COMPUTER. CAPABILITY-BASED ADDRESSING IS SEEN AS A PRACTICAL WAY TO SUPPORT FUTURE REQUIREMENTS FOR SECURITY AND RELIABLE SOFTWARE WITHOUT SACRIFICING REQUIREMENTS FOR PERFORMANCE, FLEXIBILITY, AND SHARING. A FIVE-PAGE LIST OF REFERENCES IS INCLUDED. (AUTHOR ABSTRACT)...ELW