The present state of the theory of the interactions of high-energy hadrons, photons, and leptons with nuclei is reviewed. Emphasis is placed on evidence for the quark-parton structure of hadrons in inelastic collisions with nuclei and on effects resulting from an increase in the secondary-particle formation lengths. The discussion is based on the space-time picture of strong interactions which follows from the multiperipheral nature of high-energy inelastic reactions.