Constellation-X is NASA's next major X-ray astronomy mission. Along with LISA, it is a flagship mission of NASA's Beyond Einstein program. Constellation-X is to provide a substantial improvement in sensitivity for X-ray spectroscopy, that will lead to revolutionary advances in our understanding of the physics in regions of high gravity (black holes and neutron stars) and of the major constituents of the universe (dark energy, dark matter, and the missing baryons).