The XMM-Newton mission – ESA’s X-ray observatory – which was launched in 1999 and continues to provide the worldwide astronomical community with an unprecedented combination of imaging and spectroscopic X-ray capabilities, together with simultaneous optical and ultra-violet images, using the European Photon Imaging Camera (EPIC), the Reflection Grating Spectrometer (RGS), and the Optical Monitor (OM) will be introduced. The EPIC consists of three CCD detectors; two of these are MOS (Metal Oxide Semi-conductor) CCD arrays. They are installed behind the X-ray telescopes that are equipped with the gratings of the Reflection Grating Spectrometers (RGS). The gratings divert about half of the telescope incident flux towards the RGS detectors such that about 44% of the original incoming flux reaches the MOS cameras. The third X-ray telescope has an unobstructed beam and uses pn CCDs. The EPIC cameras offer the possibility to perform extremely sensitive imaging observations over the telescope’s field of view of 30 arcmin and in the energy range from 0.15 to 15 keV with moderate spectral (E/Delta E ~ 20-50) and angular resolution (15 arcsec HEW, point spread function). The peak effective are of all three cameras combined is around 2500 cm2 at around 1.5 keV. All the EPIC CCDs operate in photon counting mode with a fixed, mode dependent frame read-out frequency, producing event lists, i.e. tables with one entry line per received event. Each of the two RGS instruments consists of an array of reflection gratings, which diffract X-rays onto an array of dedicated CCD detectors. The RGS instruments achieve high resolving power (150 to 800) over a range from 5 to 35 Å (0.33 to 2.5 keV) (in the first spectral order). The effective area peaks around 15 Å (0.83 keV) (first order) at about 150 cm2 for the two spectrometers. The OM uses a 30 cm diameter Ritchey-Chretien telescope to provide coverage between 170 nm and 650 nm of the central 17 arc minute square region of the X-ray field of view, permitting routine multiwavelength observations of XMM targets simultaneously in the X-ray and ultraviolet/optical bands.

XMM-Newton was originally approved for 2.25 years of operations with a design lifetime of 10 years. Operations have been extended via a series of rolling extensions until 31 December 2022, with further extensions possible. The design of the spacecraft and its instruments requires continuous real-time supervision at the Mission Operations Centre at ESOC and at the Science Operations Centre (SOC) at ESAC. The SOC is responsible for running the annual Call for Observing Proposals. These typically result in an oversubscription of the available time by a factor more than 5 and involve more than 1500 astronomers, worldwide. ESAC also hosts the XMM-Newton Scientific Archive, which contains a treasure trove of undiscovered surprises. The ground segment also includes the Survey Science Consortium, a multi-national consortium which together with the instruments’ Principal Investigator teams continue to provide valuable expertise and support.

XMM-Newton is contributing to almost all aspects of modern astronomy. The scientific impact is truly impressive with more than 6500 refereed papers so far. These cover topics ranging from the solar system, exoplanets, stars of all kinds, supernovae, galactic black holes, AGN, clusters of galaxies, the Warm Hot Intergalactic Medium (WHIM) to cosmology and most things in between. A main focus of XMM-Newton exoplanet research is the determination of the X-ray/UV environments of exoplanets and the study of their impact on exoplanets atmospheres. XMM-Newton has continued to shed new light on various aspects of Ultra Luminous X-ray sources (ULXs) and their physical mechanisms through detailed observations, often together with other observatories. XMM-Newton discovered quasi-periodic oscillations from the AGN GSN 069 which became 50 times stronger in X-rays for about one hour every nine hours. No AGN has ever behaved so predictably. Another major breakthrough was the direct measure of a BH spin and mass through X-ray reverberation mapping of the AGN IRAS 13224‐3809 which revealed a dynamic view of the inner accretion region and allowed the breaking of inherent degeneracies. The uncertainty on the blackhole mass is comparable to the leading optical reverberation method. An important result is the clear detection of absorption features associated with the WHIM. A deep XMM-Newton observing campaign of a quasar provided the required RGS spectrum. This result helps to finally resolve the “missing baryons” problem, i.e. the observed number of baryons in the local universe falls far short of the total number of baryons predicted by Big-Bang Nucleosynthesis.