“A Ponzi scheme”, “a will-o’-the-wisp”, “a bubble”. As the prices of the largest crypto-currencies halved in the early months of this year, after a spectacular rise in 2017, regulators made no secret of their qualms. No surprise, then, that crypto-regulation is on the agenda when finance ministers from the world’s 20 largest economies meet in Buenos Aires over the next two days. As most concur that crypto-currencies do not yet pose a risk to the banking system, the meeting will focus on the role of virtual currencies in enabling money-laundering, tax evasion and the financing of terrorism. Here a multilateral approach could help. The Financial Action Task Force, an inter-governmental body set up to combat illicit financial activities, points out that the current patchwork of national rules can be easily exploited by criminals. This much is easy to agree on, but getting to a common set of rules will take more than two days.