STS-108 was a Space Shuttle mission to the International Space Station (ISS) flown by Space Shuttle Endeavour. Its primary objective was to deliver supplies to and help maintain the ISS. STS-108 was the 12th shuttle flight to visit the International Space Station and the first since the installation of the Russian airlock called Pirs on the station. Endeavour delivered the Expedition 4 crew to the orbital outpost. The Expedition 3 crew returned to Earth on Endeavour.