Book excerpt: This paper analyzes long-term exchange rate modeling. The paper reviews the literature that tests for a unit root in real exchange rates and the closely related work on testing for a unit root in the residual from a regression of the nominal exchange rate on relative prices. It argues that the balance of evidence is supportive of the existence of some form of long-term exchange rate relationship. The paper highlights that the form of this relationship, however, does not accord exactly with a traditional representation of the long-term exchange rate.