Galaxy clusters, the largest and most massive well-defined structures known, are powerful X-ray emitters. Space-based X-ray telescopes provide the most detailed look at the ~100 million Kelvin gas in clusters. We use our observations to study physical processes such as cluster collisions or the interaction of cluster gas with outbursts from central supermassive black holes, and also to constrain the amount of dark matter in the Universe and the nature of dark energy. We are scientifically involved in important new and planned X-ray satellite missions that will revolutionize our understanding of the hot and energetic universe.