Heating and Cooling of Molecular Clouds and Their Surfaces

Some of the basic mechanisms of heating molecular clouds and their surfaces are reviewed, including cosmic rays, ambipolar diffusion, radiation, and shock waves. Cooling by fine structure transitions of atoms and ions as well as by vibrational and rotational transitions of interstellar molecules is discussed. A theoretical application of these processes is made to the observations of warm (T ≡ 200K) CO and vibrationally excited H₂.