Do you need a passport to travel from the US to Canada and back?

You should unless you're a minor.

Yes.

Ian M

Yes. That's the law since last year.

http://travel.state.gov/travel/cbpmc/cbpmc_2223.html

yahh you doo

If you are travelling by air, definately. You don't need a passport for travelling by car as of yet, just a government issued ID, such as a drivers license, military ID etc. As of the summer of 2009, that will change and a passport will be required for any form of travel.

If you are flying, yes.

If you are driving, you only need government-issued ID (driver's license) and proof of citizenship (birth certificate).

This will change next year.