delta. -nucleus interactions. CTP No. 887

The theoretical description of pion-nucleus interactions, specifically in the intermediate-energy region dominated ..delta..-excitation, is discussed. Attention is focused on the interaction with and propagation through the nucleus of pions and ..delta..'s. The approach followed basically consists of viewing the ..delta..-nucleus system, formed by pion excitation of a nucleon, as a doorway for the pion-nucleus reaction. Analysis of the interacting ..delta..-nucleus system then leads to new dynamical information summarized in a complex ..delta..-nucleus interaction potential. The associated nuclear medium corrections to ..delta.. propagation, related in more conventional language to higher-order corrections in the pion optical potential, are so large as to demand consideration in quantitative studies of nuclear structure with pions. After a discussion of the basic scale parameters characterizing intermediate-energy pion-nucleus interactions, the author discusses the theoretical framework and the extraction of ..delta.. interaction parameters from inclusive pion-nucleus cross sections. More detailed tests of the picture, such as elastic scattering angular distributions, are then discussed. Finally, some implications for intermediate-energy photoreactions are outlined. 11 figures. (RWR