Paul Ewald's 1916-1917 masterpiece `On the Foundations of Crystal Optics' describes the selfconsistent interaction of electromagnetic waves with crystals on a molecular level. While astonishing in its detailed predictions for X-ray diffraction, full appreciation of the theory and its utility had to await much later advances in measurement techniques and crystal growth. Concepts introduced in the theory, now known as the Ewald sphere, Ewald summation, and Ewald-Oseen extinction, have become mainstays in diverse areas of modern physics.This memorial volume, with contributions by leading figures in the field, is an affectionate survey of his life, scientific work, and role in the international crystallographic community.