UPDATED INFORMATION AS OF JUNE 1st, 2009 regarding Passport requirements for Canada: 

Air travel to and from Canada requires a US passport book for everyone of all ages, unless you have a valid NEXUS card. NEXUS card holders can use their NEXUS card as a substitute for a passport provided you are flying to preclearance Canadian airport with NEXUS kiosk.

Land and sea travel requires a passport, passport card, NEXUS, FAST or SENTRI card in most cases.

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