Molecules and dust in circumstellar shells

Circumstellar shells provide a unique environment for the study of dust formation and the relation of dust composition to specific atomic and molecular components. As a specific example, the formation of carbonaceous dust is discussed in relation to the presence of polycyclic aromatic hydrocarbon molecules and their survival in the interstellar medium. Some conclusions will be drawn concerning the composition of carbonaceous dust in circumstellar sources and that in the diffuse interstellar medium.