  The interstellar medium is characterized by a rich and diverse chemistry.
Many of its complex organic molecules are proposed to form through radical
chemistry in icy grain mantles. Radicals form readily when interstellar ices
(composed of water and other volatiles) are exposed to UV photons and other
sources of dissociative radiation, and, if sufficiently mobile, the radicals
can react to form larger, more complex molecules. The resulting complex organic
molecules (COMs) accompany star and planet formation, and may eventually seed
the origins of life on nascent planets. Experiments of increasing
sophistication have demonstrated that known interstellar COMs as well as the
prebiotically interesting amino acids can form through ice photochemistry. We
review these experiments and discuss the qualitative and quantitative kinetic
and mechanistic constraints they have provided. We finally compare the effects
of UV radiation with those of three other potential sources of radical
production and chemistry in interstellar ices: electrons, ions and X-rays.
