The International X‐ray Observatory (IXO) is a joint ESA‐JAXA‐NASA effort to address fundamental and timely questions in astrophysics: What happens close to a black hole? How did supermassive black holes grow? How does large scale structure form? What is the connection between these processes? To address these questions IXO will employ optics with 3 sq m collecting area and 5 arc sec angular resolution—20 times more collecting area at 1 keV than any previous X‐ray observatory. Focal plane instruments will deliver a 100‐fold increase in effective area for high‐resolution spectroscopy, deep spectral imaging over a wide field of view, unprecedented polarimetric sensitivity, microsecond spectroscopic timing, and high count rate capability. The mission is being planned for launch in 2021 to an L2 orbit, with a five‐year lifetime and consumables for 10 years.