Bottom Line: Travellers from all countries, including the U.S., must show valid a passport or equivalent travel document to enter Canada by air.
Visitors from certain countries also require a visa to visit or transit through Canada.
Best Advice: If you are still unsure about the passport requirements for your flight to Canada, consult the airline with which you are travelling. Passengers must comply with the airline's regulations, which will be spelled out on their website.
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In Depth: Although the Canada Border Services does not technically require U.S. citizens have an American passport to enter Canada by air, the Western Hemisphere Travel Initiative (WHTI) - a U.S. plan introduced in 2004 to increase border security - does require U.S. citizens to present their passports upon re-entry. In this way, the U.S. and Canada border laws may be different on paper, but, in practice, the same because the airlines will require passengers show a valid passport or other valid travel document: no sense taking someone somewhere if they can't return home again.
For more information, see
Canada Border Services Agency OR U.S. citizens should consult the U.S. Department of State.
Further Reading about the Canadian Border:
Tips for Driving across the Border into Canada
Duty Free Shopping at the Canadian Border
Bringing Children across the Canadian Border
How Much Liquor Can I Buy at the Border?