One century ago this month, highenergy electromagnetic radiation—the “X” or “unknown” ray—was discovered by Wilhelm Conrad Rontgen in his basement laboratory. Few other scientific discoveries were as immediately sensational. From hospitals to airports, in physics and biology labs, in the fabrication of nanostructures for electronics and machinery, x rays have come to permeate the modern world. In this special issue we cannot look at the full history and extent of research using x rays. Rather, we have chosen to take a few snapshots of research then and now.