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This paper presents the di¤erential approach to applied demand analysis. The demand systems of this approach are general, having coe ¢ cients which are not neces-sarily constant. We consider the Rotterdam parameterization of di¤erential demand systems and derive the absolute and relative price versions of the Rotterdam model, due to Theil (1965) and Barten (1966). We address estimation issues and point out that, unlike most parametric and semi-nonparametric demand systems, the Rotterdam model is econometrically regular. JEL classi\u85cation: D12, E2