Our current knowledge on the chemical composition of interstellar ices is summarized with respect to the possible contribution of energetic photons and particles to their observed state. We describe the inventory of astronomical ices as determined by infrared observations from the Infrared Space Observatory (ISO) and laboratory spectroscopy. Sources of radiolysis, UV photolysis, and ice heating are then discussed in the context of the chemical state of material in dense molecular clouds and in protostellar cores. Through specific examples we show how energetic processing can explain the observed solid-state characteristics of several key molecules: CH3OH, CO2, and OCN-. We also discuss the gaseous and solid-state photochemistry of the first organic acid detected in interstellar ices, HCOOH. ¿ 2001 American Geophysical Union