Progress (in Russian: "Прогресс") MS-21 is a Russian cargo vehicle which will resupply the International Space Station. This will be the 174th flight of a Progress.

It was the first cargo to fly in space, and also the first to bring freight back to Earth, thanks to a Raduga capsule.

It's a cargo ship developed to supply the Salyut 6 space station, which was subsequently used successively to supply the crews staying on board the Salyut 7, Mir and from the International Space Station. It made its first flight in 1978 and was the first vessel of this type: it enabled the crews to stay in space by bringing consumables (food, water, fuel, oxygen) and spare parts.

The Progress spacecraft is largely derived from the Soyuz spacecraft intended for the transport of crews in low orbit. It is launched by a Soyuz rocket lifting off from the Baikonur cosmodrome. It has a mass of around 7 tonnes for a length of 7.9 meters and its carrying capacity is around 2.5 tonnes. It can transport both pressurized freight and gases, propellants and liquids but is not designed to bring freight back to Earth (except with a Raduga capsule). Like the Soyuz spacecraft, it is equipped with a Kours automatic docking system. Several variants of the Progress spacecraft have been developed over the decades with increasing capabilities.