Macroeconomic models of nominal exchange rates perform poorly. The propor-tion of monthly exchange rate changes that these models can explain is essen-tially zero. This paper presents a model of a new kind. Instead of relying exclu-sively on macroeconomic determinants, the model includes a determinant from the field of microstructure?order flow. Order flow is the proximate determinant of price in all microstructure models. This is a radically different approach to ex-change rate determination. It is also strikingly successful in accounting for real-ized rates. Our model of daily changes in log exchange rates produces R 2 statis-tics above 60 percent. For the DM/$ spot market, we find that $1 billion of net dollar purchases increases the DM price of a dollar by about 0.5 percent.