You need an eTA if you are travelling to Canada by air from a visa-exempt country. A full list of visa-exempt countries is available here.
The following are exempt from requiring an eTA:
- Those who have already obtained a valid visa to enter Canada.
- Anyone entering by land or sea.
- Armed Forces visiting Canada on official duty.
- Students who have a valid Canadian study permit dated on or after August 1, 2015.
- Flight crew, civil aviation inspectors, accident investigators.
- Temporary Foreign Workers who have a valid Canadian work permit date on or after August 1, 2015.
- French citizens who live in and are travelling from St. Pierre and Miquelon.
- Persons entering from the United States or St. Pierre and Miquelon, while holding valid status in Canada.
- Accredited diplomats.
- Canadian citizens and dual citizens (provided you are travelling on a Canadian passport)
- Canadian permanent residents (you need your PR card or point of entry documentation)
- US citizens (US permanent residents, or green card holders, DO require an eTA when travelling by air)