While you are not strictly "required" to check in online for most full-service airlines, it is highly recommended, and for many low-cost carriers (like Ryanair or Spirit), it is effectively mandatory to avoid heavy fees. Checking in online, usually available 24 to 48 hours before departure, allows you to confirm your seat, add baggage, and receive a digital boarding pass on your phone. If you skip this step, many budget airlines will charge you an "airport check-in fee" that can be as high as $50–$70. For international travel, checking in online also allows the airline to verify your travel documents (passport and visas) in advance, which can save you significant time at the airport. If you have checked luggage, you will still need to visit a "bag drop" counter, but if you only have a carry-on and a digital boarding pass, you can head straight to security. Even for airlines that offer free airport check-in, doing it online ensures you get a better choice of seats and a faster experience at the terminal.