The Operating Growth Budget at Yale University is a model used to indicate what would happen to the university's various endowment funds over a substantial period of time, under verying conditions of both university policy and the business cycle. The role of the return from endowment in the operating budget can thus be calculated for yearly planning purposes. The model is discussed in terms of (1) its context in the other budgets at Yale, (2) the rationale leading to its development, (3) its mechanism, and (4) the uses to which it has been put. The model is built around 12 simultaneous differential equations. The inputs include (1) initial values of state variables such as market values of the various fund classes, book values, and realized gains, (2) flow variables such as gifts to endowment and the yearly operating budget, and (3) system parameters such as rate of return, yield rate, and trade rate. The outputs include market and book values of funds, realized and unrealized gains, and capital gains appropriations. (HW)