CHEOPS (CHaracterizing ExOPlanets Satellite) is the first ESA Small Mission as part of the ESA Cosmic Vision program 2015-2025. The mission was formally adopted in early February 2014 with a planned launch readiness end of 2017. The mission lead is performed in a partnership between Switzerland, led by the University of Bern, and the European Space Agency with important contributions from Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden, and the United Kingdom. The mission is dedicated to searching for exoplanetary transits by performing ultrahigh precision photometry on bright starts already known to host planets whose mass has been already estimated through ground based observations. The instrument is an optical Ritchey-Chretien telescope of 30 cm clear aperture using a single CCD detector. The optical system is designed to image a de-focused PSF onto the focal plane with very stringent stability and straylight rejection requirements providing a FoV of 0.32 degrees full cone. The system design is adapted to meet the top-level science requirements, which ask for a photometric precision of 20ppm, in 6 hours integration time, on transit measurements of G5 dwarf stars with V-band magnitudes in the range 6≤V≤9 mag. Additionally they ask for a photometric precision of 85 ppm in 3 hours integration time of Neptune-size planets transiting K-type dwarf stars with V-band magnitudes as faint as V=12 mag. Given the demanding schedule and cost constrains, the mission relies mostly on components with flight heritage for the platform as well as for the payload components. Nevertheless, several new developments are integrated into the design as for example the telescope structure and the very low noise, high stability CCD front end electronics. The instrument and mission have gone through critical design review in fall 2015 / spring 2016. This paper describes the current instrument and mission design with a focus on the instrument. It outlines the technical challenges and selected design implementation. Based on the current status, the instrument noise budget is presented including the current best estimate for instrument performance. The current instrument design meets the science requirements and mass and power margins are adequate for the current development status.