Marketing managers make decisions about price, advertising, promotion, and other marketing variables on the basis of factual data, judgments, and assumptions about how the market works. BRANDAID is a flexible, on-line model for assembling these elements to describe the market and evaluate strategies. This paper motivates the model and presents its mathematics. The structure is modular so that individual decision areas can be added or deleted at will. The model is of the aggregate response type, in which decision variables relate closely to specific sales performance measures. The major submodels are advertising, promotion, price, salesmen, and retail distribution. The advertising submodel employs a long-run sales response to advertising function and a linear lag process. Promotional effects are built up from a characteristic time pattern for the type of promotion and a response curve. Salesman affect sales through a response process structurally similar to that for advertising. Retail distribution variables are intermediaries that the company affects and that in turn affect customer response. Submodel outputs combine multiplicatively. Competition enters in a modular, symmetric way through a matrix of competitive coefficients that determine the source of sales for each brand as it seeks to increase its market position.