American expats are still required to file a US federal tax return to the IRS. As expats also have to comply with the tax rules in the country where they live, it’s counterintuitive but nonetheless important that they file US taxes too. Taxing US citizens abroad, or Citizenship (rather than Residence) Based Taxation, dates back to the Civil War, but until recently the IRS was powerless to enforce expat taxes, so few expats filed.