<div class="htmlview paragraph">IASI instrument is an earth observation instrument which will be launched on the MetOp meteorological polar platform. IASI mission consists of providing vertical profiles of temperature and humidity, land and sea surface temperature, gases monitoring and cloud radiation.</div> <div class="htmlview paragraph">From a thermal point of view, the challenges in IASI design are to keep the optical pathway spatially stable, to maintain the optics and their supports to a constant temperature level and to remove the high power dissipated by the electronic boxes. Moreover, the instrument thermal control shall guarantee well controlled environments to many subsystems. Among them, IASI includes a very accurate black body reference (BBC), a three-stage cryogenic passive radiator at 100K (CBS), an infrared camera (IIS) using a microbolometer and three complex dissipative mechanisms (SCAU,CCFD,CD).</div> <div class="htmlview paragraph">To achieve these aims, an automatic P.I. active thermal control algorithm is used in association with a passive thermal control system. The separation of IASI instrument into several enclosures, thermally independent and controlled by passive and P.I. active elements, enables meeting the requirements.</div>