An air services agreement (also known as the ATA or ASA) is a bilateral agreement that allows international commercial air services between signatories. Bilateral agreements and agreements allow the sharing of the airworthiness certificate for civil aviation products between two countries. The bilateral system is based on the Chicago Convention and related multilateral treaties. The Chicago Convention was signed in December 1944 and has governed international air services ever since. the convention also contains a number of annexes covering issues such as aviation safety, safety monitoring, seaworthiness, navigation, environmental protection and facilities (acceleration and departure at airports). One of the first AAS after World War II was the Bermuda Agreement, signed in 1946 by the United Kingdom and the United States. The characteristics of this agreement have become models for the thousands of agreements that were to follow, although in recent decades some of the traditional clauses of these agreements have been amended (or “liberalized”) in accordance with the “open skies” policy of some governments, particularly the United States.  These agreements will ensure the continuity of agreements with the United States, Canada, Brazil and Japan when the United Kingdom leaves the European Union. Air Services Agreements (ASAs) are formal contracts between countries – Memorandums of Understanding (Memorandum of Understanding) and formal diplomatic notes.
It is not mandatory to have an ASA for the operation of international services, but cases where contract-free services exist are rare. In 1913, a bilateral exchange of notes  between Germany and France was signed in the first agreement to provide airship services. United Kingdom/Montenegro: the agreement complementing the European Extradition Convention [CS Montenegro No.1/2019] has just been published in the series of countries. In principle, all ASAs should be registered by the DAGMAR International Civil Aviation Organization of the ICAO air agreements and agreements database, but this source is not absolutely complete. A bilateral aviation security agreement (BASA), a Memorandum of Understanding or Working Agreement (AV) and related implementation procedures provide for technical cooperation between national civil aviation authorities.