Although income taxes are common—almost every country in the world has one— wealth taxes are not. It is time to change that. Over the past four decades all around the globe, the share of the national income going to owners of capital has been rising. Correspondingly, inequality in income has increased dramatically; inequality in wealth, even more so. In the US, for example, recent research shows that the share of wealth held by the top 0.1% has more than tripled since 1978. In 2012, it stood at 22%, roughly the same as the share held by the bottom 90%. The key to addressing such rising inequality lies in global cooperation. If all nations were to adopt a harmonized wealth tax, the problem of capital flight, the concern that haunts policymakers and stymies efforts to establish national wealth taxes, would be avoided. Not only would that reduce inequality, but it would also give nations a new source of revenue for financing the transition to a more just, equitable, and sustainable world.