An airport concessionaire can finance the construction, operation and maintenance of major airports from revenues that it receives from: (a) user fees, (b) landing and parking fees from airlines, (c) non-aeronautical revenue such as retail and parking, and (d) if the contract so provides, city-side development. There are two ways to keep costs low for passengers: (1) taxes can support airports such that passengers are shielded from user charges, or (2) to avoid unexpected and sudden changes in user fees, such fees may be stated upfront for the duration of the concession period (possibly indexed to inflation) prior to bidding.