United States citizens need a valid U.S. passport to fly to or transit through a Canadian airport. Citizens of the United States coming to Canada by car, bus, train or boat need to carry proper identification and meet the basic requirements to enter Canada. Citizens of the United States who are also Canadian citizens should bring a valid Canadian passport and a valid U.S. passport or travel document to board a flight from the Unites States to Canada. Carrying both documents may help simplify both your entry into Canada and your return to the United States. Permanent residents of the United States need a valid Alien Registration Card and a passport to enter Canada. Find out more information on the Government of Canada's website.