Carbon is one of the principal candidates for the light elements in Earth's core. The content and chemical bonding environments of carbon in the core are essential for understanding the nature and dynamics of the core. This chapter focuses on mineral physics investigations of density and sound velocities of candidate iron carbides and assesses the role of carbon in accounting for the core density deficit and sound velocity discrepancy. It then reviews cosmochemical and geochemical constraints on carbon in the deep Earth from the compositions of meteorite and terrestrial samples, followed by experimental results on the solubility of carbon in Fe-Ni liquids and the processes of transporting carbon to the core during the coreformation differentiation in the early history of Earth. Finally, the chapter discusses the carbon inventory in Earth's core and the implications for the deep carbon cycle in Earth's interior.