Defense planners spend considerable time generating and evaluating options for modernizing our nation’s aging fleets of weapon systems. In the U.S. Army, this process involves generating plans for modernizing units with new equipment and identifying efficient strategies for using older equipment. Dynamic constraints and variables such as changes in defense doctrine or military force structure can affect these plans. Other constraints are administrative or political in nature, such as retirement policies or budgeted procurement quantities. Key parameters can change daily, and the decision maker rarely has the ability or time to specify what kind of solution is optimal or most desirable. Selecting an appropriate modernization formula is largely an iterative process of evaluating the trade-offs between real-life resource constraints and the effect on the organizations that own the systems. The Army’s Training and Doctrine Command (TRADOC) automated the fleet-modernization and resource-distribution planning process by developing a series of object-oriented models that simulate the fleet-modernization process. Using a Lisp machine environment and rapid prototyping programming techniques, we built a deterministic framework to model and project the long-range implications of fleet-modernization decisions. By incorporating an interactive graphic user interface, we are able to support the iterative process of generating modernization plans by tracking all important decision variables and allowing the user to graphically see the effect that a plan will have on the organizations and fleets being modernized. We present how this methodology was successfully applied to two problem domains: aviation and armor modernization planning.