I will discuss key atomic and molecular processes in cometary atmospheres. Like comets in our solar system, it will be difficult if not impossible to directly study the physical and chemical properties of comets around other stars. Instead, we have to infer these properties from the gas surrounding them. Atomic and molecular reaction such as dissociation, ionization, and charge exchange both alter gases surrounding comets. Because many reactions result in the emission of light, they also offer insight into the composition and radiation environment exocomets are exposed to.