On most modern airlines, flying standby occurs when a passenger travels on a flight without a prior reservation for that specific flight. There are two circumstances in which passengers typically fly standby. First, a missed flight may require a passenger to fly standby on the next flight to the same destination, as they now lack a reservation. Secondly, a passenger may arrive at the airport early (whether accidentally or on purpose) and wish to take an earlier flight listed for that day. They will then attempt to travel standby on the earlier flight, and failing that, proceed to take their booked flight. This is referred to in the industry as "go-show". Standby can also occur for upgrades. Many airlines (particularly in the United States) give free space available domestic upgrades to First Class for their elite tier fliers. If first class sells out or upgrades full with higher-tiered passengers, elite fliers can standby for a first class seat, should one open up due to a cancellation, no-show, misconnect, irregular operations or an equipment change. If a passenger clears for an upgrade, they may be given new boarding passes at the gate. Some airlines, such as American and United, have gateside monitors that show the upgrade (as well as general) standby list, and will also announce when First Class checks in full (i.e., no further upgrades will be given).

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