The Kh-55 (Russian: ?-55[note 1], also known as RKV-500; NATO reporting name: AS-15 "Kent") is a Soviet/Russian subsonic air-launched cruise missile, designed by MKB Raduga in the 1970s. It has a range of up to 2,500 km (1,350 nmi) and can carry nuclear warheads. Kh-55 is launched exclusively from bomber aircraft and has spawned a number of conventionally armed variants mainly for tactical use, such as the Kh-65SE and Kh-SD, but only the Kh-101 and Kh-555 appear to have been put into service. Contrary to popular belief, the Kh-55 was not the basis of the submarine and ground-launched S-10 Granat or RK-55 Relief (SS-N-21 "Sampson" and SSC-X-4 "Slingshot") designed by NPO Novator. The RK-55 is very similar to the air-launched Kh-55 (AS-15 "Kent") but the Kh-55 has a drop-down turbofan engine and was designed by MKB Raduga.

In the late 1960s, the "Ekho" study conducted by the GosNIIAS institute concluded that it would be more effective to deploy many small, subsonic cruise missiles than the much more expensive supersonic missiles then in favour.[8] Work started at the Raduga bureau on an air-launched cruise missile in 1971, with a first test flight in 1976.[9] The appearance of the US Air Force's AGM-86 ALCM in that year gave further impetus to the programme, with the Soviet Air Force issuing a formal requirement for a new air-launched cruise missile in December 1976.[8] The longer-range Kh-55SM was developed a few years after the original went into service. In the late 1980s work began on a replacement missile with either conventional (Kh-101/X-101) or nuclear (Kh-102) warheads[7] and greater stealth. It was designed by Igor Seleznyev of Raduga.[5] The importance of advanced missiles as "force multipliers" increased as Russia's fleet of available cruise-missile bombers declined in the early 1990s.[10] The cancellation of the ambitious Kh-90 ramjet missile due to INF Treaty in 1987 led to a renewed emphasis on improving the Kh-55, in particular to achieve the <20 m a