Automatic two-month filing extension
If your tax home is overseas (that is, you have met the requirements of either the Bona Fide or Physical Presence tests) then you are automatically eligible for a two-month filing extension, normally to June 16.
This extension allows you to file and send your payment without facing penalties for late payment. However, if you do make a payment, you are still required to pay for interest dating from April 15 (interest is calculated at a fixed percentage rate, while penalties, designed to be punitive, are higher).
In order to claim this extension, you must add a simple declaration to your tax packet when you send it in. It must state that (A) you currently live outside of the United States and Puerto Rico, and (B) your tax home for the tax year in question is outside of the United States and Puerto Rico. For example:
I, John Flendersen, SSN 123-45-6789, am claiming the automatic two-month filing extension on the basis that I currently live in Madrid, Spain, and my tax home, as established by the Bona Fide residency test, has been in Spain since 2006.
If you require an extension beyond the two months in order to satisfy one of the residency tests (e.g., you will satisfy the Physical Presence test in August) then you must complete Form 2350, Application for Extension of Time to File.