The ESA Science Programme Committee (SPC) selected CHEOPS (Characterizing Exoplanets Satellite) in October 2012 as the first Small-class mission (S1) within the Agency’s Scientific Programme, with the following 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. The CHEOPS mission 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. It 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 instrument is an optical Ritchey-Chrétien telescope with 300 mm effective aperture diameter and a large external baffle to minimize straylight. The compact CHEOPS spacecraft (approx. 300 kg, 1.5 m size), based on a flight-proven platform, 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 the course of 2017, flight platform and payload have been integrated and tested, while satellite level activities are planned to start in early 2018, targeting flight readiness by the end of the year. The paper describes the latest CHEOPS development status, focusing on the acceptance test performed on instrument and platform, as well as on the satellite level environmental test campaign.