The Constellation Program was a human spaceflight program developed within NASA, the space agency of the United States, from 2005 to 2009. The milestone goals of the program were "completion of the International Space Station" and a "return to the moon no later than 2020" with the planet Mars as the ultimate goal.

The program's logo reflected the three stages of the program—earth (ISS), moon, Mars—while the Mars goal found expression in the name given to the program's booster rockets: Ares. Technological aims of the program included the regaining of significant astronaut experience beyond low earth orbit and development of technologies necessary to enable sustained human presence on other planetary bodies.

Constellation began in response to the goals laid out in the Vision for Space Exploration under NASA Administrator Sean O'Keefe. It had already begun development, under several proposals. O'Keefe's successor, Michael D. Griffin, ordered a complete review, termed the Exploration Systems Architecture Study, which reshaped how NASA would pursue the goals laid out in the Vision for Space Exploration. The NASA Authorization Act of 2005 formalized the findings of the Exploration Systems Architecture Study. The Act directed NASA to "develop a sustained human presence on the Moon, including a robust precursor program to promote exploration, science, commerce and US preeminence in space, and as a stepping stone to future exploration of Mars and other destinations." Work began on this revised Constellation Program to send astronauts first to the International Space Station, then to the Moon, then Mars and beyond.