Since the Great Financial Crisis, several scandals have exposed a pervasive light on banks' presence in tax havens. Taking advantage of a new database, this paper provides a quantitative assessment of the importance of tax havens in international banking activity. Using comprehensive individual country-by-country reporting from the largest banks in the European Union, we provide several new insights: 1) The average eect of being a tax haven is an extra presence of foreign aliates by 168%; 2) For EU banks, the main tax havens are located within Europe: Luxembourg, Isle of Man and Guernsey rank at the top; 3) Attractive tax rates are not sucient to drive extra activity; 4) But lenient regulatory environment attract extra commercial presence; 4) Banks avoid the most opaque countries with weak governance; 5) The tax savings for EU banks is estimated between €0.7 billion and €3.8 billion.