While “aggressive tax planning” is conceptually elusive, tackling it does not require elaborate definitions. Much can be achieved by taxing income flows within multinational firms, to the extent that these flows go from high-tax to low-tax jurisdictions. Such a “tax on aggressive tax planning” can take the form of a conditional withholding tax on intragroup payments of interest and royalties. Assuming a rate of 15%, the tax is only applied if the intragroup payment is not effectively being taxed at that same rate in the recipient’s state of residence. We argue that a conditional withholding tax helps to counter base erosion and profit shifting and sets an international minimum norm for corporate tax rates. We also find that such a tax is not incompatible with EU law.