Yield curve models within the popular Nelson and Siegel (hereafter NS) class are shown to arise from a formal low-order Taylor approximation to the generic Gaussian a¢ ne term structure model. That theoretical foundation provides an assurance that NS models correspond to a well-accepted framework for yield curve modeling. It further suggests that any yield curve from the GATSM class should be parsimoniously representable by an two-factor arbitrage-free NS model, which should prove useful for macro…nance applications. Such a model is derived and applied to provide evidence for changes in United States yield curve dynamics pre-and post-1988. JEL: E43, G12, C58 Keywords: yield curve; term structure of interest rates; Nelson and Siegel model; a¢ ne term structure models.