Airlines often sell more tickets than the seats which are available for certain flights. This vicious practice causes a big hassle for passengers, because when booking or purchasing a ticket, the ticketing system issues a completely valid ticket or reservation. In such cases if you show up on time for the check-in and there are no available seats on board, the flight crew will deny you access to the board of the aircraft. If that is the case, the airline owes you a compensation.
Often airlines offer vouchers, special flight points or petty money to the passengers, who are willing to give up their seats voluntarily and fly later on. In case of such voluntary acts in favour of another passenger, you will not be entitled to a cash compensation for denied boarding.
If none of the passengers give up their seat in your favour, the airline will be forced to rebook you for a later flight or even with another airline, at their expense. In this case, in spite of providing you with another free flight, the airline still owes you a cash compensation of up to €600 for the inconvenience that they have caused you.
Passengers who have been denied boarding because they have traveled with an invalid passport or without a visa (if one was required) are not entitled to a cash compensation for denied access to the board of the aircraft.