When layover flights are booked with the same airline, your baggage will be automatically transferred through to your final destination. However, if the two flights are with different airlines, you may have to claim and re-check your baggage during your layover.

Whether your bags are transferred automatically between different airlines depends entirely on how your tickets were booked. If you have a "through-ticket" (a single booking reference or PNR) involving partner airlines—such as flying United and then switching to Lufthansa within the Star Alliance—your bags will almost always be "checked through" to your final destination. You will receive all your boarding passes at the first check-in, and the airline's ground crew handles the transfer. However, if you have booked "self-transfer" flights (two separate tickets on unrelated airlines like Ryanair and then Delta), you must collect your bags at the transit airport, clear customs/immigration, and re-check them at the second airline's desk. This is a crucial distinction, as self-transfers require much longer layovers. There are exceptions: when entering the United States on an international flight, you must always collect your bags at your first point of entry to clear Customs, even if they are tagged through to your final destination; you then simply place them on a "re-check" belt immediately after the customs hall.