The economy is globalizing, but the tax system remains fragmented across two hundred countries, opening ample opportunity for the wealthy to evade paying their fair share. A number of “tax haven” nations welcome foreign-owned assets, keeping depositors’ information confidential and thus making it difficult to determine the magnitude of this shielded wealth. However, a recent study provides the first comprehensive estimate: a staggering $7.6 trillion, twice the GDP of Germany. This total grew by roughly 25% over the past five years, and, without international coordination, it will only continue to grow. The benefits of cooperation are clear: taxing the uncovered assets at current rates would generate nearly $200 billion annually, new revenue that could be applied to countering inequality and climate change. The transnational flow of money demands transnational regulation.