The AIM-9 Sidewinder is a short-range air-to-air missile. It entered service with the United States Navy in 1956 and the Air Force in 1964, and is one of the oldest, cheapest, and most successful air-to-air missiles. Its latest variants (AIM-9X) remain standard equipment in most Western-aligned air forces. The Soviet K-13 (AA-2 "Atoll"), a reverse-engineered copy of the AIM-9B, was also widely adopted.