This paper is neither a complete survey of empirical work on exchange rate determination, nor a review of the ballooning volume of theoretical models. It is instead an attempt to classify the main alternative approaches to modeling exchange rates. I shall concentrate on approaches that can be used to assess the effects of alternative policies. There will be four further sections in the paper. The first three sections will each deal withthe structure, empirical support, and policy consequences of three main types of model: purchasing power parity models emphasizing the close and immediate relation of goods markets; interest rate parity models emphasizing the close and immediate international linkage of markets for financial assets; and structural balance-of-payments models that do not assume either of the above linkages to be so strong and immediate as to eliminate the other, and that hence require separate (but interdependent) modeling of trade and capital linkages in the determination of exchange rates. Each of these main categories has many rather distinct models within it, and some models are not easily classified into one of the three categories; I hope that the three-way split will nevertheless serve to make some distinctions that are important for policy modeling. In the final section I shall try to summarize the available model results that pertain to national and international policy choices under a system of more flexible exchange rates, and then to suggest where more or better model building might usefully increase the amount of information available to guide policy decisions.