We use the 2003 National Survey of College Graduates to provide the first estimates of the effect on earnings of having a double major. Overall, double majoring increases earnings by 2.3% relative to having a single major among college graduates without graduate degrees. Most of the gains from having a double major come from choosing fields across two different major categories. Graduates who combine an arts, humanities or social science major with a major in business, engineering, science or math have returns 7-50% higher than graduates with a single major in arts, humanities or social science. But such double major combinations have returns no higher than single majors in business, engineering, science or math. Majors combining business and science or math have returns more than 50% greater than the returns to having a single major in these fields. (Contains 5 tables.)