I applied and got an ESTA last year I cannot remember whether there was paper work; I cannot find any. Can I visit with just my passport? Same question about the Canada eTA.
Answer for the US ESTA:
Whether you have an ESTA or not will only be checked during the check-in process with your airline. Once you arrived in the US, you do not need to show that you have an ESTA. The immigration authorities can look this up in their computer if necessary.
Many airlines have an automatic process to check for the existence of a valid ESTA registered with your passport details on check-in. Hence, you then do not need to show any printouts or the like. However, not all airlines have this, and hence, it is sometimes necessary to have a print-out of the approval confirmation with you when you travel to the US.
ESTAs and eTAs are essentially electronic passport registrations; hence, you only need your passport.
A few airport handling agents may not be able to verify your ESTA/eTA, however, in which case you should print the confirmation page (or present it on the phone).
If you want to be sure, you can email the handling agent in advance of your trip.