Book excerpt: This book describes the state of the art in the field of modeling and solving numerically inverse problems of wave propagation and diffraction. It addresses mathematicians, physicists and engineers as well. Applications in such fields as acoustics, optics, and geophysics are emphasized. Of special interest are the contributions to two and three dimensional problems without reducing symmetries. Topics treated are the obstacle problem, scattering by classical media, and scattering by distributed media.