If you are a citizen of the United States, you do not need a passport to enter Canada or the U.S. However, you should carry proof of your citizenship, such as a birth certificate, certificate of citizenship or naturalization, as well as a photo ID. If you are a permanent resident of the U.S., you should bring your Permanent Resident Card (i.e. green card) with you.
U.S. citizens returning to the U.S. from Canada are currently exempt from passport requirements.
For information visit www.travel.state.gov or www.cbsa.gc.ca