  In this chapter, we provide a review of radiative processes in cometary
atmospheres spanning a broad range of wavelengths, from radio to X-rays. We
focus on spectral modeling, observational opportunities, and anticipated
challenges in the interpretation of new observations, based on our current
understanding of the atomic and molecular processes occurring in the
atmospheres of small, icy bodies. Close to the surface, comets possess a
thermalized atmosphere that traces the irregular shape of the nucleus. Gravity
is too low to retain the gas, which flows out to form a large, collisionless
exosphere (coma) that interacts with the heliospheric radiation environment. As
such, cometary comae represent conditions that are familiar in the context of
planetary atmosphere studies. However, the outer comae are tenuous, with
densities lower than those found in vacuum chambers on Earth. Comets,
therefore, provide us with unique natural laboratories that can be understood
using state-of-the-art theoretical treatments of the relevant microphysical
processes. Radiative processes offer direct diagnostics of the local physical
conditions, as well as the macroscopic coma properties.These can be used to
improve our understanding of comets and other astrophysical environments such
as icy moons and the interstellar medium.

Earth and Planetary Astrophysics