Solutions of the d'Alembert equation in ?n.- Solutions of the d'Alembert equation in arbitrary domains.- Steady-state scattering theory in exterior domains and the limiting absorption principle.- Time-dependent scattering theory in exterior domains.- Steady-state scattering theory and eigenfunction expansions for A.- Wave operators and asymptotic solutions of the d'Alembert equation in exterior domains.- Asymptotic wave functions and energy distributions in exterior domains.