CHEOPS (Characterizing Exoplanets Satellite) is devoted to the characterization of known exoplanets orbiting bright stars, achieved through the precise measurement of exoplanet radii using the technique of transit photometry. CHEOPS was selected in October 2012 as the first Small-class mission (S1) within the Agency’s Scientific Programme, with the following programmatic requirements: science driven mission selected through an open Call; an implementation cycle, from the Call to launch, drastically shorter than for Medium-class (M) and Large-class (L) missions; a strict cost-cap to ESA, with possibly higher Member States involvement than for M or L missions. Following a phase A/B1 study, CHEOPS was adopted for implementation in February 2014 as a partnership between the ESA Science Programme and Switzerland, with a number of other Member States delivering significant contributions to the instrument development and to operations. The CHEOPS payload is a high precision photometer, with an optical Ritchey-Chrétien telescope with 300 mm effective aperture and a large external baffle to minimize straylight. The CHEOPS spacecraft (280 kg mass, 1.5 m size) is based on a flight-proven platform and will orbit the Earth in a dawn-dusk Sun Synchronous Orbit at 700 km altitude. CHEOPS completed the Preliminary Design Review at the end of September 2014, and passed the Critical Design Review in May 2016. In 2017, flight platform and payload have been separately integrated and tested, while satellite activities were completed by end 2018, allowing to reach flight readiness. CHEOPS is scheduled for launch on a shared Soyuz flight by the end of 2019.