Theoretical investigations of the electromagnetic interactions of electrons, ions, atoms, and molecules with each other and with photons are carried out both analytically and numerically. There are research programs concerned with intense radiation fields leading to multiphoton excitation and ionization, with collision theory, with relativistic photon scattering, radiation, and absorption, and with modification of these processes in hot and/or dense plasma environments. These studies have been important applications to x-ray laser development, to laser and magnetic fusion, to atmospheric and space physics, and to astrophysics.