If you are visiting us...

From the United States... American visitors crossing the border
(either way) may be asked to verify their citizenship with a
document such as a passport, or a birth or baptismal certificate.
Naturalized U.S. citizens should carry a naturalization certificate.
Permanent US residents who are not citizens are advised to bring
their Alien Registration Receipt Card (Green Card).

And young travellers... Travellers under the age of 18, if not
accompanied by a parent or legal guardian, must carry a letter
of permission to travel in Canada, signed by a parent or guardian.

From all other countries... Citizens of all other countries, except
Greenland and residents of St. Pierre et Miquelon, must bring a
valid passport. Some may be required to obtain a visitor's visa:
for details, please consult the Canadian embassy or consulate
serving your home country.