Cryptocurrencies provide an important dimension of innovation to the evolution of the exchange medium we call money. There are now over 2,000 such currencies, and their potential and volume is growing. How- ever, they will, collectively and in volume, create real problems for the monetary system of a country. Central banks, which are institutions tasked with providing monetary stability, are more essential than ever. Yet they will see their problems rise while the power of their traditional tools to control money supply and interest rates—such as reserve requirements and the discount rates—is declining. But the new digital technologies— such as distributed ledgers—and new approaches provide regulatory bodies also with new and potentially powerful tools. The task for central banks and policy makers is not to resist private digital currencies as troublesome irritants, but to create approaches to use, regulate, and incent them in shaping the macro-economic path of their economy. In the process, central banks will also issue their own digital currencies, and a small number of those will become global super-currencies.