This study developed a model to explain monthly imports of butter and butter substitutes. The U.S. imports butter and other high milk fat products that can substitute for a strict definition of butter. These products include dairy spreads, butter substitutes, anhydrous milk fat, and food preparations. The U.S. imports these high milk fat products under a tariff-rate quota system (TRQ) implemented by the World Trade Organization (WTO). This study quantified all the milk fat contained in U.S. butter and butter substitute imports. A conceptual model was developed to explain imports of these products under a TRQ system. And an econometric model was estimated to analyze the economic factors that drive over-quota imports. The model found that the wedge between U.S. and world butter prices explains much of the incentive to import overquota butter and butter substitutes.