This article provides a theoretical economic foundation for the popular Nelson and Siegel (1987) class of yield curve models (which has been absent up to now). This foundation also offers a new framework for investigating and interpreting the relationships between the yield curve, output and inflation that have already been well-established empirically in the literature. Specifically, the level of the yield curve as measured by the VAO model is predicted to have a cointegrating relationship with inflation, and the shape of the yield curve as measured by the VAO model is predicted to correspond to the profile (that is, timing and magnitude) of future changes in the output gap (that is, output growth less the growth in potential output). These relationships are confirmed in the empirical analysis on 50 years of United States data.