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Summary form only given. This talk provides a 20-year perspective on the use of analytic models to design of a wide range of commercially important architectures and systems with complex behavior. These systems include resources with highly bursty and/or correlated packet arrivals, communication protocols with complex routing and blocking of messages, resources that are configured for a very high probability (e.g., 0.9999) of providing immediate service to each arriving client, and complex large-scale grid/Internet applications. The examples illustrate some guiding principles for model development, and show that the models can be relatively easy to develop. More importantly, the models can be highly accurate - often more accurate than simulation, and sometimes more accurate than the system implementation! The examples also illustrate that the models can provide unique insight into system design as well as significant new system functionality. In other words, analytic models are a key tool for competitive systems engineering. Time permitting, the talk includes some important observations about workload models, and some ways to avoid key pitfalls in simulation.