(excerpted from http://travel.state.gov/travel/cis_pa_tw/cis/cis_1082.html#entry_requirements, September 1st, 2011)

ENTRY/EXIT REQUIREMENTS: A visa is not required for U.S. citizens to visit Canada for up to 180 days. Anyone seeking to enter Canada for any purpose other than a visit (e.g. to work, study or immigrate) must qualify for the appropriate entry status, and should contact the Canadian Embassy or nearest consulate and consult the Canadian immigration website.

Entry into Canada is solely determined by Canadian Border Services Agency (CBSA) officials in accordance with Canadian law.  Please see the CBSA's website for details.  Canadian law requires that all persons entering Canada carry both proof of citizenship and proof of identity. A valid U.S. passport, passport card or NEXUS card (see below) satisfies these requirements for U.S. citizens. Children under 18 must present proof of citizenship and identity.  The CBSA website provides further details about bringing children into Canada.  (Please see below for important information concerning re-entry into the United States).  For further information about visiting Canada, consult the Citizenship and Immigration Canada's Visiting Canada website.

U.S. citizens entering Canada from a third country must have a valid U.S. passport.

If you plan to travel to Canada with a minor who is not your own child or for whom you do not have full legal custody, CBSA may require you to present a notarized affidavit of consent from the minor's parents.  Please refer to the CBSA website for more details.  There is no specific form for this document, but it should include dates of travel, parents' names and photo copies of their state-issued IDs.  

When returning to the United States from Canada, it is very important to note that all U.S. citizens are required to present a valid U.S. passport to enter or re-enter the United States via air.  For entry into the United States via land and sea borders, U.S. citizens will need to present either a U.S. passport, passport card, NEXUS card, Enhanced Drivers License or other Western Hemisphere Travel Initiative (WHTI)-compliant document.  The only exception to this requirement is for U.S. citizens under the age of 16 (or under 19, if traveling with a school, religious, or other youth group) who need only present a birth certificate (original, photocopy or certified copy), Consular Report of Birth Abroad, or naturalization certificate. 

American travelers are urged to obtain WHTI-compliant documents before entering Canada well in advance of their planned travel.  For the most recent information on WHTI and WHTI-compliant documents, please see theU.S. Department of State's WHTI website or the Customs and Border Patrol website.

Both the U.S. and Canadian governments urge frequent travelers to join the NEXUS trusted traveler program.  The NEXUS Card is for pre-approved travelers who cross the U.S/Canada border frequently. It has added benefits of access to expedited passage at dedicated commuter lanes on the northern border, expedited marine reporting and access to NEXUS kiosks at designated Canadian airports. To learn more about this program or to apply for entry into this program, please refer to the CBP.GOV website./p>

Please Note: Canada, like the United States, has rules that prohibit entry for persons with specific ineligibilities.  For example, anyone with a criminal record (including misdemeanors or Driving While Impaired (DWI)), no matter how long ago, may be barred from entering Canada and must obtain a special waiver well in advance of any planned travel. To determine whether you may be inadmissible and how to overcome this finding, please refer to the Citizenship and Immigration Canada website.

For further information on entry requirements, travelers may contact the Canadian Embassyat 501 Pennsylvania Avenue NW, Washington D.C. 20001, tel. (202) 682-1740; or the Canadian consulates in Atlanta, Boston, Buffalo, Chicago, Dallas, Detroit, Los Angeles, Miami, Minneapolis, New York, San Juan or Seattle.

Information about dual nationality or the prevention of international child abduction can be found on our website. For further information about customs regulations, please read our Customs Information sheet.