The Aircraft Warning Service (AWS) was a civilian service of the United States Army Ground Observer Corps instated during World War II to keep watch for enemy planes entering American airspace. It became inactive on May 29, 1944.

During World War I, the airplane was not generally used as a long-range fighting machine. However, during the period from 1919 to the start of World War II, the heavy bomber was created, capable of ranging far from its home base and carrying a lethal load of high explosives. It soon became clear that a warning system was needed to protect against this new threat. Technology at the outset of World War II consisted of mechanical sound detectors that were found to be inadequate to the job. It was also argued that while soldier lookouts would be valuable, their use would detract from other needed military operations.^{[better source needed]}

With the help of the American Legion, volunteers were organized in May 1941 into the Aircraft Warning Service, the civilian arm of the Army's Ground Observer Corps. On the east coast, the AWS was under the auspices of the Army Air Force's 1st Interceptor Command (later First Fighter Command or I Fighter Command) based at Mitchel Field, New York. On the west coast, the AWS was under the auspices of the 4th Interceptor Command (later Fourth Fighter Command or IV Fighter Command) based in Riverside, California. On both coasts, observation posts, information centers and filter centers were established. The United States entered World War II on Dec 7, 1941. Many of those who could not join the military for whatever reason were recruited to the AWS. Statistically, this led to a preponderance of women.

All observers received training in aircraft recognition. This training spilled over into the non-AWS population. Aircraft recognition became a hobby and many books and publications resulted on the subject. Many participated in contests and recognition "Bees". Recognition clubs and meetings were set up. Black, hard rubber, spotter models were made for various aircraft.

As the war escalated, thousands of observation posts were established on the east coast from the top of Maine to the tip of Florida, and roughly inland as far as the western slopes of the Appalachian Mountains. On the west coast, posts ranged from upper Washington to lower California. Each post had its own code name and number. When aircraft were spotted, the volunteers recorded their observations on forms or in log books and then quickly place a call to a regional Army Filter Center and verbally deliver a "Flash Message" which contained the organized data from the observation. In California these messages were a series of brief phrases describing the number of planes, estimated altitude, heading direction, and number of engines: e.g., "one, high, south east, four" <personal observation>. One can imagine that aircraft approaching the coast would be spotted by multiple posts, resulting in multiple Flash Messages and, therefore, a reasonably accurate triangulation of position, speed, direction, altitude, etc.

The training and intense watching bore fruit in the autumn of 1943 when observers at a post in West Palm Beach, Florida, saw and reported instantly the passage of a German aircraft bearing American markings over their post. The fact that the plane was one that had been captured in Europe and was being flown back to the United States for examination detracted not one bit from the effectiveness of the recognition instruction.

At the peak of operation, the Aircraft Warning Service of the First Fighter Command numbered some 750,000 people, of whom about 12,000 were in information and filter centers. Practice interceptions, run almost daily, provided experience for ground officers and pilots working under simulated combat conditions. Because of its huge scope of operations on the East Coast, the AWS saved the lives of many pilots by bringing speedy aid to those whose planes had crashed.

A detailed account of life in an east coast observation post is offered by Meade Minnigerode in his book, Essex Post, published in 1944 by the Yale University Press.