Remote sensing of atmosphere is changing rapidly thanks to the development of high spectral resolution infrared space- born sensors. The aim is to provide more and more accurate information on the lower atmosphere, as requested by the World Meteorological Organization (WMO), to improve reliability and time span of weather forecasts plus Earth's monitoring. In this paper the performance of the Infrared Atmospheric Sounding Interferometer (IASI) is analyzed looking directly at the products: temperature and water vapor.