One can now attempt to determine the abundances of key species in the atmospheres of exoplanets. In parallel, the knowledge of the densities of these exoplanets inform us on their bulk composition in terms of amounts of dense material (rocks and ices) compared to light ones (hydrogen and helium). Linking these constraints seems natural and, intuitively, one would expect dense planets to contain more heavy elements in their atmospheres. However, several physical processes, in particular the formation of a central core and the growth of a deep outer radiative zone, should decouple partially or even completely interior and atmospheric composition. Depending on how heavy elements were delivered to the planets, their atmospheres may have very different compositions. I will discuss how transit surveys like CoRoT and subsequent spectroscopic observations have improved our view of the compositions of giant exoplanets. I will compare these results to what is known about giant planets in our solar system and will discuss how Juno’s arrival in orbit around Jupiter in July 2016 should provide detailed constraints on its internal structure and composition.