2012 marked the centenary of one of the most significant discoveries of the early twentieth century, the discovery of X-ray diffraction (March 1912, by Laue, Friedrich, and Knipping) and of Bragg's law (November 1912). The discovery of X-ray diffraction confirmed the wave nature of X-rays and the space-lattice hypothesis. It had two major consequences: the analysis of the structure of atoms, and the determination of the atomic structure of materials. The momentous impact of the discovery in the fields of chemistry, physics, mineralogy, material science, biochemistry and biotechnology has been recognized by the General Assembly of the United Nations by establishing 2014 as the International Year of Crystallography. This book relates the discovery itself, the early days of X-ray crystallography, and the way the news of the discovery spread round the world. It explains how the first crystal structures were determined, and recounts which were the early applications of X-ray crystallography. It also tells how the concept of space lattice has developed since ancient times, and how our understanding of the nature of light has changed over time. The contributions of the main actors of the story, prior to the discovery, at the time of the discovery and immediately afterwards, are described through their writings and are put into the context of the time, accompanied by brief biographical details.