This comprehensive introduction to an important area of nuclear physics summarizes the developments in the field over the last decade and provides essential theoretical background. Particular emphasis is placed on the closed formalism for strong-absorption processes. Elastic and inelastic scattering, transfer reactions, and coupled-channel extensions are covered. This book, found among Frahn's papers after his death, represents his final thoughts on the subject to which he devoted his life's work. As such, it is invaluable for advanced students and researchers in nuclear physics.