Europe is one of the major partners building the International Space Station (ISS) and European industry. together with ESA, is responsible for many station components including the Columbus Orbital Facility. the Automated Transport vehicle. two connecting modules and the European Robotic Arm. Together with this impressive fist of contributions there is a strong desire within the ESA Member States to benefit from this investment by utilizing the unique capabilities of the ISS to perform world-class science. XEUS is one of the astronomical applications being studied by ESA to utilize the capabilities of the ISS. XEUS will be a long-term X-ray observatory with an initial mirror area of 6 m(2) at 1 keV that will be expanded to 30 in 2 following a visit to the ISS. The I keV spatial resolution is expected to be 2-5" half-energy-width. XEUS will consist of separate detector and mirror spacecraft (MSC) aligned by active control to provide a focal length of 50 m. A new detector spacecraft, complete with the next generation of instruments, will also be added after visiting the ISS. The limiting 0.1-2.5 keV sensitivity will then be 4 x 10(-18) erg cm(-2) s(-1), around 200 times better than XMM-Newton, allowing XEUS to study the properties of the hot baryons and dark matter at high redshift. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.