X-ray astronomy comes of age

The Chandra X-ray Observatory (Chandra) and the X-ray Multi-Mirror Mission (XMM-Newton) continue to expand the frontiers of knowledge about high-energy processes in the Universe. These groundbreaking observatories lead an X-ray astronomy revolution: revealing the physical processes and extreme conditions involved in producing cosmic X-rays in objects ranging in size from a few kilometres (comets) to millions of light years (clusters of galaxies), and particle densities ranging over 20 orders of magnitude. In probing matter under conditions far outside those accessible from Earth, they have a central role in the quest to understand our place in the Universe and the fundamental laws that govern our existence. Chandra and XMM-Newton are also part of a larger picture wherein advances in subarcsecond imaging and high-resolution spectroscopy across a wide range of wavelengths combine to provide a more complete picture of the phenomena under investigation. As these missions mature, deeper observations and larger samples further expand our knowledge, and new phenomena and collaborations with new facilities forge exciting, often unexpected discoveries. This Review provides the highlights of a wide range of studies, including auroral activity on Jupiter, cosmic-ray acceleration in supernova remnants, colliding neutron stars, missing baryons in low-density hot plasma, and supermassive black holes formed less than a billion years after the Big Bang.