Requirements depend on your citizenship, destination, transit countries, and travel dates. Travel with the passport and any visas or authorizations required for your itinerary. Check official government guidance well before departure, including passport validity requirements. A passport copy does not replace your original passport.
For U.S. citizens visiting the Schengen Area, the passport generally must remain valid for at least three months after the planned departure from that area and have been issued within the previous ten years. Maintaining at least six months of validity is a useful planning precaution, but it is not a universal legal requirement for all destinations.