Expired U.S. passports
Section 215 of the Immigration and Nationality Act (8 U.S.C. 1185) states:
Except as otherwise provided by the President and subject to such limitations and exceptions as the President may authorize and prescribe, it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport.
The term "United States" as used in this section includes the Canal Zone, and all territory and waters, continental or insular, subject to the jurisdiction of the United States. The term "person" as used in this section shall be deemed to mean any individual, partnership, association, company, or other incorporated body of individuals, or corporation, or body politic."
This means you must ensure you have a valid U.S. passport for travel from and to the United States at all times. For dual nationals it means they are not to travel on the passport of their other nationality, even though that passport would normally allow for visa-free travel under the Visa Waiver Program. You must travel on your U.S. passport instead.
Unlike a lost or stolen passport, an expired passport does not constitute an emergency that normally warrants the issuance of an emergency passport (see our lost/stolen passport page and our imminent departure page).
For passport application instructions, click here.