The composition of a comet nucleus cannot be directly determined by remote sensing observations, but is usually inferred from coma observations, assuming certain conditions in the near-nucleus environment. The development of cometary activity is undoubtedly related to most complex, yet unknown, physico-chemical processes in the surface layer of the nucleus and the inner coma. Hence, it is presently not possible to unambiguously extract the true composition of a comet nucleus from its coma composition. Key information on these processes is expected from the Rosetta mission to comet 67P/Churyumov-Gerasimenko. Rosetta will monitor the evolution of the nucleus surface and the coma composition as a function of increasing and decreasing solar flux input along the comet’s pre- and postperihelion orbit. Different instrumentations will be used in parallel, from multi-wavelength spectrometry to in-situ measurements of coma and nucleus composition and physical properties. This provides the opportunity to cross-correlate the values obtained by the different measurement techniques on board, and in addition link the inner coma chemistry uncovered by the space mission to the abundance of coma species observed from Earth or Earth orbit. Thus, the Rosetta target comet can serve as a reference for interpretation and modelling of the physical and chemical processes relevant for the formation of a comertary coma. If this knowledge is properly transferred to other comets, it will provide new means to distinguish whether differences between comets, observed in their comae, reflect differences of their nuclei or are related to different environmental conditions of the comets at the time of observation. This in turn has important implications to the question of how and where comets were formed in the early solar system.