The OMI instrument is an ultraviolet-visible imaging spectrograph that uses two-dimensional CCD detectors to register both the spectrum and the swath perpendicular to the flight direction with a 115° wide swath, which enables global daily ground coverage with high spatial resolution. This paper presents a selection of in-flight radiometric and CCD detector calibration and performance monitoring results since the launch in July 2004. From these examples it will be shown that OMI is performing very well after more than four years in orbit. It is shown how the OMI irradiance measurement data have been used to derive a high resolution solar reference spectrum with good radiometric calibration, good wavelength calibration and high spectral sampling. The surface reflectance climatology derived from three years of in-orbit OMI measurement data is presented and discussed. The OMI mission may possibly be extended in 2009 for another two or four years, depending on the performance of the instrument. By 2013-2014 OMI on EOS-Aura and SCIAMACHY on ENVISAT will have reached more that twice their anticipated lifetimes. In order to guarantee continuity of Earth atmosphere tropospheric and climate measurement data new instrumentation shall be available around that time. A successor of OMI and SCIAMACHY, named TROPOspheric Monitoring Instrument (TROPOMI), scheduled for launch by the end of 2013, is discussed in this paper.