Giant planets are by definition mainly made of hydrogen and helium but their precise composition is crucial to understand the processes that led to their formation, and to constrain the origins of the Solar System. Interestingly, all giant planets in the Solar System are enriched in heavy elements compared to the Sun. This is true of the planets as a whole, of their envelopes (i.e. when removing their central core) and of their atmospheres as well. It is now becoming possible to constrain the global composition of transiting extrasolar planets too, with three major results: (i) Planets above the mass of Saturn appear indeed to be mainly made of hydrogen and helium, as expected; (ii) Surprisingly, some planets were able to collect large masses (> 50 M⊕) of heavy elements; (iii) There is a correlation between the mass of heavy elements in a given giant planet and the metallicity of its parent star. These results and others points towards accretion being the dominant mode of formation of giant planets. However there is presently no unique process or ensemble of processes that account for the observed compositions of giant planets.