A bilateral air transport agreement (also sometimes called a bilateral air service agreement or ATA or ASA) is an agreement which two nations sign to allow international commercial air transport services between their territories.
In 1913, in what was probably the earliest such agreement, a bilateral Exchange of Notes was signed between Germany and France to provide for airship services.
One of the first post-Second World War ATAs was the Bermuda Agreement, which was signed in 1946 by the United Kingdom and the United States. Features of this agreement became models for the thousands of such agreements that were to follow, although in recent decades some of the traditional clauses in such agreements have been modified (or "liberalized") in accordance with "open skies" policies adopted by some governments, notably the United States [1].
In principle all ATAs should be registered by the International Civil Aviation Organization in DAGMAR but this source is not absolutely comprehensive.