On most modern airlines, flying standby is when a passenger without a seat assignment waits at the gate to see if there is an extra seat after all scheduled passengers have boarded. There are several common circumstances in which passengers fly standby:

• A prospective passenger is not booked on the flight, but waits to see if there is an extra seat after all scheduled passengers have boarded.
• A missed flight requires a passenger to fly standby on the next flight to the same destination, as they now lack a reservation.
• A passenger who is already booked on a flight arrives at the airport early (accidentally or deliberately) and asks to be on standby for an earlier flight. If a standby seat doesn't open up, they just take their booked flight. The industry calls this a go-show.
• A ticketed passenger requests to stand by for an upgrade. Many airlines, particularly in the United States, give free space-available domestic upgrades to first class to their elite tier fliers. If first class sells out or upgrades go to higher-tiered passengers, elite fliers can stand by in the event a first-class seat becomes available due to a cancellation, no-show, misconnect, irregular operation, or equipment change. If a passenger clears for an upgrade, they may receive a new boarding pass at the gate. Some airlines, such as American and United, have gate-side monitors that show the upgrade and general standby list, and announce when first class is full (no further upgrades are available).
• If a flight is overbooked, an airline may designate all passengers without a seat assignment as "standby" prior to boarding.