To enter Canada, you must present a valid passport, passport card or WHTI-compliant document (see link in References for a complete list). Also, an enhanced driver's license (available in four states as of March 2010) may be used for identification when crossing into Canada.
Children under the age of 15 traveling with an adult must have one of the above documents as well as a birth certificate, U.S. consular report of birth abroad or a Certificate of U.S. Naturalization.
You may be required to answer specific questions for the Canadian Border Services Agency (CBSA) when crossing the border into Canada. These inquiries may be about your residence, purpose of visit, length of stay and what you're carrying with you.
If the border agent feels your responses are unsatisfactory---especially with regard to identity, firearms, tobacco and alcohol---regulations dictate that they can ask you to report to the main customs building for further questioning and subject your vehicle to a search.
If you are a single parent traveling with a child, you may be required to produce a letter indicating that you are the sole guardian. A child traveling with people other than their parents may need letter of permission for travel signed by both parents. For verification, these documents should be notarized.
Criminal and DUI (driving under the influence of drugs or alcohol) conviction records can prevent you from being permitted to cross the border into Canada. Regulations may dictate that the border authorities check these records, and recent convictions are a permissible reason to deny entry.
Canadian law does not permit travelers to carry firearms or weapons into Canada. Regulations require border authorities to question travelers about whether or not they own or are carrying firearms or weapons. If you are carrying any weapons or firearms, they will be confiscated and you will not be permitted to cross the border.
