Constellation Program (abbreviated CxP) is a human spaceflight program within NASA, the space agency of the United States. The stated goals of the program are gaining significant experience in operating away from Earth's environment, developing technologies needed for opening the space frontier and conducting fundamental science.

Constellation was developed through the Exploration Systems Architecture Study, which determined how NASA would pursue the goals laid out in the Vision for Space Exploration and the NASA Authorization Act of 2005. On February 1, 2010, President Obama announced a proposal to cancel the program effective with the U.S. 2011 fiscal year budget.

Part of the Constellation program involves the development of spacecraft and booster vehicles to replace the Space Shuttle and send astronauts to the Moon and possibly to Mars as well. NASA has already begun the process of designing two boosters, the Ares I and Ares V. Ares I would have the sole function of launching mission crews into orbit, while Ares V would be used to launch other hardware for use on missions and will have a heavier lift capacity than the Ares I booster. In addition to these two boosters, NASA is designing a set of other spacecraft for use during Constellation. These will include the Orion crew capsule, the Earth Departure Stage and the Altair lunar lander.

A number of alternatives to Constellation have been suggested in response to critiques of the program. In 2009, concerned by price increases on the program, U.S. President Barack Obama ordered a review of the project which indicated that both returning to the Moon and manned flights to Mars were out of NASA's current budget. The Augustine panel decided to advocate proceeding with the Ares V only, with subsequent modifications to transport astronauts when needed, rather than constructing both the Ares I (for crew transport only) and the Ares V (for cargo only).

Advocates for the Constellation program, in testimony given to Congress, pointed out that launching the enormous Ares V for merely transporting people (to the ISS for example) would be prohibitively costly: "As I see it, the commission didn't find anything wrong with the current program, didn't find anything safer, more reliable, cheaper or faster," former NASA Administrator Michael D. Griffin has testified to Congress.  "The roots are healthy. So why throw away four years and $8 billion pulling the flowers? Let's apply some plant nutrient and watch them grow.... Exactly why does the policy which we established in law — twice — need to be changed?" According to Griffin, NASA spending has declined approximately 20% since 1993.

Debates persist regarding whether the present program should continue, be discontinued, or rectified in some way.