Radiobiology is in a state of rapid change, and at such a time intensive theoretical work is needed. The present symposium was arranged for a meeting of the Radiation Research Society; it contains 5 papers covering the domain from macromolecules to cells. The papers illustrate the two-sided nature of theoretical radiobiology: what is known about the organization of living things determines the analysis of how radiation effects are propagated through the organism, and what is known about the propagation of radiation effects help to elucidate the pattern of organization.