Energetic processing of interstellar ice mantles and planetary atmospheres via photochemistry is a critical mechanism in the extraterrestrial synthesis of prebiotic molecules. Photochemistry is defined by chemical processes initiated by photon-induced electronic excitation, not involving ionization. In contrast, photons with energies above the ionization threshold initiate radiation chemistry (radiolysis). Vacuum-ultraviolet (6.2–12.4 eV) light may initiate photochemistry and radiation chemistry because the threshold for producing secondary electrons is lower in the condensed phase than in the gas phase. Approximately half of cosmic-ray induced photons incident on interstellar ices in star-forming regions initiate pure photochemistry, while the rest can also initiate radiation chemistry. While experimental techniques such as velocity map imaging may be used to extract exquisite details about gas-phase photochemistry, such detailed information is more difficult to obtain for condensed-phase photochemistry, which involves greater complexity, including the production of excitons, excimers, and exciplexes. Because a primary objective of chemistry is to provide molecular-level mechanistic explanations for macroscopic phenomena, our ultimate goal in this book chapter is to critically evaluate our current understanding of the photochemistry that likely leads to the synthesis of extraterrestrial prebiotic molecules.