The 2K12 Kub mobile surface-to-air missile (SAM) system is a Soviet low to medium-level air defence system designed to protect ground forces from air attack. "2К12" is the GRAU designation of the system. Its NATO reporting name is SA-6 Gainful.

Each battery consists of a number of similar tracked vehicles, one of which carries the 1S91 (NATO designation Straight Flush) 25 kW G\H band radar (range 75 km\47 miles) equipped with a continuous wave illuminator, in addition to an optical sight.

The battery usually also includes four triple-missile transporter erector launchers (TELs) and four trucks each carrying three spare missiles and a crane.

The SA-6 system shares a lot of components with the SA-4 system. In many ways they are designed to complement each other; SA-4 is effective at long ranges and high altitudes, SA-6 at medium ranges and intermediate altitudes. The SA-7 and SA-8 systems complete this picture with the low-altitude and short range capabilities.

The system is able to acquire and begin tracking targets using the 1S91 "Straight Flush" radar at 75 km (47 miles) and begin illumination and guidance at 28 km (17 miles). IFF is also performed using this radar. It can only guide between one and two missiles to a single target at any time. The missile is initially command guided with terminal semi active radar homing (SARH), with target illumination provided by the "Straight Flush" radar. Detonation is via either the impact or proximity fuse. On the latest models, this vehicle is also fitted with an optical tracking system which allows engagement without the use of the radar (for active RF emissions stealth reasons, or due to heavy ECM jamming) in which case the effective altitude is limited to 14 km\46000 ft. The optical tracking method also allows engagements to altitudes below that where the radar is able to track targets. Maximum target speed is around Mach 2 for head-on engagements and Mach 1 for tail-chase engagements. Top speed of the missile is approximately Mach 2.8.

In contrast to the elaborate Patriot missile or even the simpler Hawk system fielded by US forces, most of the system rides on two tracked self-propelled vehicles, rather than towed or mounted on trucks, and either the launcher or control vehicle can be set to launch in only 15 minutes after changing location.