If the share of payments made by cryptocurrencies increases, government-issued money will face market competition from private issuers. The column argues that, even if this system could maintain price stability in an economy, the market would not provide the socially optimum amount of money. A government could still, however, maximise social welfare using monetary policy in response to peg the real value of money. The threat of competition from private monies may therefore impose welcome market discipline on any government that issues currency.