What do you do if the airline loses your luggage?

File a written claim at the airline's baggage desk before leaving the airport and keep the reference number, because pursuing a claim after departure is substantially harder and some carriers impose short reporting deadlines. Most bags are delayed rather than lost and turn up within a couple of days. During that window, US Department of Transportation rules require airlines to reimburse reasonable interim expenses for necessities, and the airline's policy on what counts is negotiable in practice, so keep every receipt. Once a bag is officially declared lost, domestic liability runs up to roughly 4,700 dollars per passenger, an amount DOT adjusts periodically, and the airline must also refund the checked bag fee.

International itineraries fall under the Montreal Convention, which sets a lower liability limit, currently around 1,700 dollars equivalent, and requires written notice within 7 days for damage and 21 days for delay. Two things improve your position materially. Photograph your packed bag and keep receipts for expensive contents, since airlines depreciate and dispute value aggressively and the burden is on you. And put an AirTag or similar tracker in checked luggage, which has repeatedly proven decisive when a carrier claims a bag's location is unknown while the passenger can see it sitting in a warehouse. Note that airlines exclude liability for many categories including electronics, jewelry, cash, and fragile items, which is the practical argument for keeping those in your carry-on rather than trusting the system.