The FBAR (FinCEN Form 114) must be filed by any US person — citizen, resident, green card holder, or certain entities — who had a financial interest in or signature authority over one or more foreign financial accounts with an aggregate value exceeding $10,000 at any point during the calendar year. "Financial accounts" include bank accounts, brokerage accounts, mutual funds, and some foreign pensions — it's a broad definition. The FBAR is filed electronically through FinCEN's BSA E-Filing System, not with your tax return, and the deadline is April 15 with an automatic extension to October 15. The penalties for willful non-filing can reach the greater of $100,000 or 50% of the account balance per violation — one of the steepest penalties in tax law. Even if you had only $10,001 in a foreign account for a single day, you're required to file.