We identify a 'slope' factor in exchange rates. High interest rate currencies load more on this slope factor than low interest rate currencies. As a result, this factor can account for most of the cross-sectional variation in average excess returns between high and low interest rate currencies. A standard, no-arbitrage model of interest rates with two factors - a country- specific factor and a global factor - can replicate these findings, provided there is sufficient heterogeneity in exposure to the global risk factor. We show that our slope factor is a global risk factor. By investing in high interest rate currencies and borrowing in low interest rate currencies, US investors load up on global risk, particularly during bad times.