It is commonly assumed that the building blocks of the terrestrial planets were derived froma cosmochemical reservoir that is best represented by chondrites, the so-called chondritic Earthmodel. This view is possibly a good approximation for refractory elements (although it hasbeen recently questioned; e.g., Caro et al. 2008), but for volatile elements, other cosmochemicalreservoirs might have contributed to Earth, such as the solar nebula gas and/or cometary matter(Owen et al. 1992; Dauphas 2003; Pepin 2006). Hence, in order to get insights into the originof the carbon in Earth, it is necessary to compare: (i) the elemental abundances and isotopiccompositions of not only carbon, but also other volatile elements in potential cosmochemical“ancestors,” and (ii) the ancestral compositions with those of terrestrial volatiles. This approachis the only one that has the potential for understanding the origin of the carbon in Earth butit has several intrinsic limitations. First, the terrestrial carbon budget is not well known, and,for the deep reservoir(s) such as the core and the lower mantle, is highly model-dependent(Dasgupta 2013; Wood et al. 2013). Second, the cosmochemical reservoir(s) that contributedvolatile elements to proto-Earth may not exist anymore because planet formation might havecompletely exhausted them (most of the mass present in the inner solar system is now in Venusand Earth). Third, planetary formation processes (accretion, differentiation, early evolutionof the atmospheres) might have drastically modified the original elemental and isotopiccompositions of the volatile elements in Earth. Despite these limitations, robust constraints onthe origin(s) of the carbon in Earth can be deduced from comparative planetology of volatileelements, which is the focus of this chapter.< Réduire