The Constellation-X mission is a large collecting area X-ray facility, emphasizing observations at high spectral resolution (R ~ 300--3000) while covering a broad energy band (0.25-60 keV). By increasing the telescope aperture and utilizing efficient spectrometers the mission will achieve a factor of 25-100 increased sensitivity over current high resolution X-ray spectroscopy missions. Constellation-X is the X-ray astronomy equivalent of large ground-based optical telescopes such as the Keck and the VLT, complementing the high spatial resolution capabilities of Chandra. Key technologies under development for the mission include lightweight high throughput X-ray optics, multilayer coatings to enhance the hard X-ray performance of X-ray optics, micro-calorimeter spectrometer arrays, low power and low weight CCD arrays, lightweight gratings and hard X-ray detectors. Constellation-X will for the first time make high resolution X-ray spectroscopy of faint X-ray source populations a matter of routine. With its increased capabilities, Constellation-X will address many fundamental astrophysics questions such as observing the formation and evolution of clusters of galaxies, constraining the Baryon content of the Universe, observing the effects of strong gravity close to the event horizon of black holes and using these effects to determine the black hole rotation. The Constellation-X mission has received strong endorsements in two recent National Academy of Sciences reports: the Astronomy and Astrophysics Survey and the Committee on the Physics of the Universe.