Heat and ionization sources in the interstellar medium

This is a review of the astrophysical sources of heat and ionization in interstellar clouds and in the intercloud medium. Observational data on density, ionization, and temperatures of H I regions are summarized, and the heat and ionization requirements are specified in terms of heat input and hydrogen ionization rate. A wide variety of sources and mechanisms are reviewed and contrasted, and corresponding estimates of heat input and hydrogen ionization rate are presented in tabular form.