A parallel R-matrix program PRMAT for electron-atom and electron-ion scattering calculations This paper describes a new parallel R-matrix program PRMAT which enables very large electron-atom and electron-ion scattering calculations of importance in many applications to start to be addressed for the first time on the current generation of massively parallel computers. The paper commences with an overview of R-matrix theory applied to electron-atom and electron-ion scattering, in which the fundamental concepts and the basic equations of the theory are given. A detailed description of this new parallel program PRMAT is then given and its relation to earlier scalar programs RMATRXII and FARM, which treat respectively the internal and external R-matrix regions, is discussed.