The recent reversal of capital flows to emerging markets raises the question of whether and how to intervene in currency markets. Brazil’s central bank has intervened heavily, spending more than $50 billion and promising to double that by the end of the year. However, almost all of that intervention has taken place in onshore derivative markets that settle in real. This column argues that such interventions can be effective, but that central banks must stand ready to use their foreign-exchange reserves if necessary.