Since the term was first used (Williamson, 1983), a large number of authors have calculated Fundamental Equilibrium Exchange Rates (hereafter FEERs). As a result FEERs have become the main rival to PPP calculations as a method of assessing whether an exchange rate is undervalued or overvalued. Despite the importance of the methodology, much less work has been done to investigate how sensitive these calculations are to exogenous assumptions, or to weaknesses in the underlying structure. This chapter aims to fill this gap.