Customer service

Check-in

How and when do I check in online?
How do I check-in for my low-cost booking (e.g. Ryanair/Laudamotion, Easyjet, Wizzair, Vueling)?
Where do I find my booking reference and/or ticket numbers?
Can I check in for a return flight already?
I am unable to check-in online. Can I check-in at the airport?