To gain insight in the dynamics and the long-term behavior of constituents (e.g., ozone, CFCs) of the Earth atmosphere, satellite-based instruments allowing global monitoring provide unsurpassed information. In order to study atmospheric trends over a very long period of time, these instruments generally have to be calibrated normalized to the sun very accurately. Furthermore, the accuracy has to be maintained during the lifetime of the instruments. This contribution addresses the issue of radiometric calibration of earth-observation instruments, both on ground and in flight. Some of the related issues will be illustrated by focusing on two specific instruments (to be) calibrated at TPD: the global ozone monitoring experiment (GOME), which was launched on ERS-2 in April 1995, and the scanning imaging absorption spectrometer for atmospheric cartography (SCIAMACHY), to be launched on Envisat around the turn of the century. We distinguish between sun-normalized calibration and absolute radiometric calibration. In both cases instrument sensitivity to polarization is a complicating factor. Other factors to be dealt with are, e.g., the etalon effect and the influence of humidity during on-ground calibration. These all require a sophisticated calibration approach and well-adapted radiometric calibration equipment. After on-ground calibration the instruments are susceptible to possible changes or degradation. Therefore, GOME and SCIAMACHY both contain a well-calibrated on-board diffuser providing an accurate reflectance standard. However, this diffuser itself is degrading with time due to contamination and radiation effects. An in-flight monitoring concept is therefore mandatory. The addressed calibration aspects are elucidated using the example of SCIAMACHY.