This chapter introduces the mathematical representations for multisystem and multiscale sustainability analyses focusing on supply and demand in food and agricultural markets at different scales. Within a computable multiscale modeling framework like SIMPLE-G, the supply and demand equations are derived based on production, trade, or food consumption theories. For each of the five successively more complex models, agricultural inputs are either environmental resources (e.g., land and water) or manufactured and human inputs (e.g., fertilizer and labor). The degree of complexity in SIMPLE-G models depends on the details about agricultural inputs, the assumptions about market connections and input mobility, the choice of functional forms, and the spatial heterogeneity required by the research question.