It’s certainly legal to have foreign bank accounts and other investments outside the US. In some cases, it’s expected: as in the case of foreign families moving to the United States or US persons doing business overseas. But beginning in 2012, the Foreign Account Tax Compliance Act (FATCA) requires US taxpayers to disclose certain foreign assets with an aggregate value exceeding $50,000. For married individuals filing jointly, this threshold increases to $100,000 on the last day of the year or $150,000 at any time during the year. Higher thresholds apply to individuals residing outside the US. See IRS Form 8938 (currently in draft). This is in addition to Form 8621 which requires information from US shareholders of passive foreign investment companies (PFIC). Both these Forms are filed with a taxpayer’s income tax return and are in addition to the Report of Foreign Bank and Financial Accounts (FBAR) filed with Treasury.