To claim EU261 (or UK261) compensation for a flight delay or cancellation, you must first confirm that your flight qualifies: it must have departed from an EU/UK airport or arrived at one on an EU/UK carrier. The delay must be over three hours at your final destination and cannot be caused by "extraordinary circumstances" like extreme weather or air traffic strikes. To start the process, gather your boarding pass and flight details, then submit a formal claim through the airline's official website. Many airlines have a dedicated "Compensation Claim" form in their help section. If the airline denies your claim, you can escalate it to a National Enforcement Body (NEB) or use a specialized "No Win, No Fee" legal service, though they will take a percentage of your payout. Compensation amounts range from €250 to €600 per passenger depending on the distance of the flight and the length of the delay.