Since the early 1990s, central banks in many emerging markets and developing countries have accumulated foreign reserves at an unprecedented rate. The macroeconomic impact of these official flows has been profound and they have contributed significantly to global imbalances. Providing an explanation for these trends remains a major puzzle in international macroeconomics, and prevailing theories based on trade or debt deliver poor empirical performance. We argue that part of this great reserve accumulation is a response to the threat of financial instability in the context of rapidly expanding financial systems, increasingly mobile capital, and exchange rate objectives. The recent turbulence in global financial markets supports this view.