Nineteen German scientists contribute seven chapters reporting on the work done during the 1990s in the Collaborative Research Program ("Schwerpunkt") of the German Science Foundation. The different theoretical tools are presented, as well as the results obtained using these methods for both simple and complicated (i.e. many-atom) systems. Coverage includes basic theory and quantum electrodynamics in strong fields; four-component Ab Initio methods for atoms, molecules and solids; relativistic quantum chemistry with pseudopotentials and transformed Hamiltonians; relativistic density functional theory; magnetic phenomena in solids; experimental and theoretical study of the chemistry of the heaviest elements; and experimental probes for relativistic effects in the chemistry of heavy d and f elements.