  Photodissociation is the dominant removal process of molecules in any region
exposed to intense ultraviolet (UV) radiation. This includes diffuse and
translucent interstellar clouds, dense photon-dominated regions, high velocity
shocks, the surface layers of protoplanetary disks, and cometary and
exoplanetary atmospheres. The rate of photodissociation depends on the cross
sections for absorption into a range of excited electronic states, as well as
on the intensity and shape of the radiation field at each position into the
region of interest. Thus, an acccurate determination of the photodissociation
rate of even a simple molecule like water or carbon monoxide involves many
detailed considerations ranging from its electronic structure to its
dissociation dynamics and the specifics of the radiation field that the
molecule is exposed to. In this review chapter, each of these steps in
determining photodissociation rates is discussed systematically and examples
are provided.
