Number 247 (Story #2), November 2, 1995 by Phillip F. Schewe and Ben Stein X RAYS WERE DISCOVERED 100 YEARS AGO BY WILHELM ROENTGEN. To celebrate this centennial, the November issue of Physics Today is given over to a series of articles about the ubiquity of x rays in various research areas. The following is a sampling. Condensed matter physics: the diffraction of x rays from a crystal has for decades provided information about the location of atoms in that crystal. More recently, the trillion-fold increase in the brilliancy offered by synchrotrons over conventional x-ray sources makes it possible to use finer beams which are needed to study very pure crystals or materials under high pressure (in both of these cases the samples are likely to be tiny). Molecular biophysics: X-ray diffraction studies helped to elucidate the structure of DNA and numerous proteins. Speeding up and digitizing this process, a Protein Data Bank has been created. This compilation of atomic-level x-ray maps of biological structures now grows at a rate of hundreds of maps per year, directly benefiting the design of new drugs. Medicine: The half million patients receiving x-ray treatments and the 300 million x-ray examinations carried out annually in the U.S. testifies to the importance of x rays in diagnosis and therapy. X-ray computed tomography (CT) technology continues to provide ever faster and sharper pictures of tumors. Eventually x ray delivery for diagnosis and therapy will be integrated into a single gantry unit. Astrophysics: Orbiting x-ray detectors now look routinely at pulsars, supernova remnants, quasars, and the flares issuing from our own sun. X-ray imaging is especially valuable in the effort to demonstrate the existence of black holes and in the study of how galaxies clump together in clusters.