Constellation-X is a set of powerful X-ray telescopes that will orbit close to each other and work in unison to observe simultaneously the same distant objects, combining their data so that they have 100 times the power of any previous single X-ray telescope. Constellation-X has been designed to perform X-ray spectroscopy with unprecedented sensitivity and spectral resolution. The measurement of large numbers of X-ray spectral lines in hot plasmas will allow astronomers to determine the flow of gas in accretion disks around black holes, in active galactic nuclei, and in binary X-ray sources, measure the population of newly-created elements in supernova remnants, and detect the influence of dark matter on the hot intergalactic medium in clusters of galaxies. The Constellation-X mission, currently under design, is a key element in NASA's Structure and Evolution of the Universe theme. Current plans envision a four-satellite constellation and a launch some time around 2015, although NASA has not yet committed to the project.