The discovery of X-rays sparked a strong reaction within the scientific community. This was further fueled by the fact that they were discovered through the study of cathode rays, the great mystery of the second half of the 19th century. Just one year after Röntgen’s discovery—in 1896—x-rays found practical application in medicine, giving rise to the field of radiography. But what is their nature? Are they waves, or are they composed of (unknown) particles? Scientists searched very intensively for answers to these questions. Their search culminated in the famous experiment conducted by Max Laue, Paul Knipping, and Walter Friedrich. This was shortly followed by the work of the Braggs—father and son—who determined the atomic structure of the first substance - the mineral halite (NaCl). Thus, scientists observed atoms for the first time and ushered in the modern era of research into matter. In this lecture, we will focus on X-ray research from 1896 to 1912.