A tribute to Paul Ewald, whose work "On the Foundations of Crystal Optics" has many modern applications. Contributions to this volume are both scientific and biographical, and five key papers by Ewald from 1913, 1968, 1969, 1979 and 1986 are included. Ewald as a great scientist and gentle person comes to life in this book, with the authors presenting some glimpses into his private life and providing a well rounded presentation of his major scientific contributions. This is a remarkable monograph, and should be a 'must-read' study for everyone interested in scattering problems.