The 16-foot-long, 8,000-pound Pirs Docking Compartment is attached to the bottom, Earth-facing port of the Zvezda Service Module. It docked to the International Space Station on 16 September 2001, and was configured during three spacewalks by the Expedition Three crew.

Pirs, also known as DC-1, was launched 14 September 2001, as ISS Assembly Mission 4R on a Russian Soyuz rocket. The Docking Compartment has two primary functions. It serves as a docking port for the docking of transport and cargo vehicles to the Space Station and as an airlock for the performance of spacewalks by two Station crewmembers using Russian Orlan spacesuits.

In addition, the Docking Compartment can transport fuel from the fuel tanks of a docked Progress resupply vehicle to either the Zvezda Service Module Integrated Propulsion System or the Zarya Functional Cargo Block. It can also transfer propellant from the Zvezda and Zarya to the propulsion system of docked vehicles – Soyuz and Progress. The docking compartment's lifetime as part of the Station is five years.