Airlines in USA, as in many other countries, allow the ease of online/web check-in for their passengers. Online check-in can be performed from their desktops, laptops or any mobile device having an internet connection. This can also be done by downloading the airline mobile application in smartphone and using it for online check-in.

Following are some of the situations in which a person is rendered ineligble for online check-in. You shall know what these situations are and must always check with the airline about the same before taking the flight.

1. Group Travel of more than a specified number of people on single itinerary.
2. Traveling with infants
3. Persons requiring wheelchairs
4. Persons requiring special assistance, such as with certain medical conditions as they have to show proper documents.
5. Persons traveling with pets, as they need to show certain documents
6. Unaccompanied Minors
7. Pregnant Women, as there is a need to prove the type and progress of pregnancy with proper documentation.
8. International travelers, though allowed online check-in, might still have to be present at airport for documentation checking. 
9. It might not be operational on codeshare flights altogether or where the first flight is operated by another airline.
10. If there are more than a specified number of segments in one itinerary. For ex: United does not allow online check in for more than 4 flight sectors in an itinerary.
11. If you have not scanned the passport previous to the flight, you might not be allowed to do online check-in. Therefore, it is advisable to first scan the passport using the airline mobile app, if this is a feature provided by airline app.
12. If you have made reservations with paper-tickets (not electronically generated), you are not allowed web check-in. 
13. Web check in might not be available for certain types of seats, such as bulkhead seats in some airlines. 
14. Those traveling with Miliary ID and not a passport would also need to check-in at the counter of airline at departure airport.