This paper describes ANSWER, the expert system responsible for monitoring AT&T’s 4ESS switches. These switches are extremely important, since they handle virtually all of AT&T’s long distance traffic. ANSWER is implemented in R++, a rule-based extension to the C++ object-oriented programming language, and is innovative because it employs both rule-based and object-oriented programming paradigms. The use of object technology in ANSWER has provided a principled way of modeling the 4ESS and of reasoning about failures within the 4ESS. This has resulted in an expert system that is more clearly organized, easily understood and maintainable than its predecessor, which was implemented using the rule-based paradigm alone. ANSWER has been deployed for more than a year and handles all 140 of AT&T’s 4ESS switches and processes over 100,000 4ESS alarms per week.