  We discuss new developments of interstellar chemistry, with particular
emphasis on the carbon chemistry. We confirm that carbon chains and cycles are
ubiquitous in the ISM and closely chemically related to ea ch other, and to
carbon. Investigation of the carbon budget in shielded and UV illuminated gas
shows that the inventory of interstellar molecules is not complete and more
complex molecules with 4 or more carbon atoms must be present. Finally we
discuss the consequences for the evolution of clouds and conclude that the
ubiquitous presence of carbon chains and cycles is not a necessary consequence
of a very young age for interstellar clouds.
