In the recent years one of the most widely discussed problem affecting major European and worldwide airports concerns the lack of runway slots to satisfy airlines’ entire demand. It is generally accepted that an efficient allocation of the existing scarce capacity, and thus of the slots, could be an instrument to manage the disproportion between demand and offer. Experts agree that the current allocation procedure adopted in the EU, in particular the grandfathering of slots, is an important cause of inefficiency in the EU aviation market. It is affirmed that market mechanisms, as secondary trading, have the potential to improve efficiency. On 30th April 2008 the Commission adopted a Communication in which for the first time slot trading is deemed as legitimate.