Gamma-ray polarimetry can provide a powerful diagnostic of the high-energy physics at work in the most extreme sources of the universe: active galactic nuclei, gamma-ray bursts, black hole binaries, pulsars, etc. The linear polarisation information of gamma-ray sources is measured using Compton scattering up to a few MeV and the conversion of high-energy photons to electron–positron pairs at higher energies. Thanks to the development of gamma-ray detectors with high position and energy resolution, several balloon-borne and satellite experiments are getting important results with this new astronomical tool.