Most space experiments produce nowadays huge amounts of data which require onboard processing, either by hardware or software, before being transmitted to ground. This newly created session is intended to present operational solutions as well as projects or ideas of onboard data reduction and compression. By data reduction we understand for example the computation of moments of velocity distribution functions measured by particle detectors, the identification of spectral lines either in emission or absorption, the computation of energy spectra of recorded wave fields, or the identification of given features in waveforms or images, ... et cetera, et cetera . Data compression concerns the efficient coding of reduced data in order to maximize the amount of transmitted data.