The chemistry of interstellar clouds

A brief overview of some of the dominant chemical processes in interstellar clouds is given. The C+-C-CO transition is discussed for both homogeneous and inhomogeneous media. An understanding of the properties of these chemical transitions is necessary for the interpretation of [C II] 158 mu m, [C I] 609 mu m and CO (J'-J) line maps. The role of geometry in the apparent distribution of simple carbon and oxygen bearing species is investigated as well. A concrete example is discussed for the S140 photon-dominated region. The chemistry of simple carbon-bearing species, as well as H2O and O-2 is described. Recent millimeter absorption line studies of HCO+ and HCN indicate that these types of observations are very sensitive to trace amounts of molecular material. Their relevance to the study of molecular material in other galaxies and cosmology is discussed.