In 1995 the Advanced Photon Source at Argonne National Laboratory near Chicago, Illinois, ended a 5-year construction program and began providing insertion-device and bending-magnet radiation in the hard x-ray spectral region for research on the structure and composition of matter. Users of the Advanced Photon Source include scientists from universities, industry, and federal laboratories. They apply advanced synchrotron x-ray technology to experiments in materials science; biological science; chemical science; agricultural science; environmental science; geoscience; atomic, molecular, and optical physics; and the development of novel synchrotron x-ray instrumentation. The Advanced Photon Source belongs to the class of third-generation synchrotron light sources, which also includes the Advanced Light Source in California, the European Synchrotron Radiation Facility in France, and the 8-GeV Super Photon Ring in Japan. All of these facilities afford new opportunities for research by extending those characteristics that make synchrotron-based x-ray beams highly useful to a broad spectrum of disciplines.