United Airlines offers various check-in options to make the process convenient for passengers. Travelers can check in online through the United Airlines website or mobile app starting 24 hours before their flight's departure time. Online check-in allows passengers to select seats, add baggage, and receive a mobile boarding pass. Alternatively, passengers can check in at the airport ticket counter or self-service kiosks, which are available at most airports. United also offers curbside check-in and priority check-in for certain passengers, such as Premier members and premium cabin travelers. Checking in early helps ensure a smoother travel experience with United Airlines.