Hastings, J.B. JACoW Publishing Geneva, Switzerland Synchrotron light sources and free-electron lasers are at the heart of the rapidly evolving and growing field of accelerator-based x-ray science. The latest developments in this field are presented. The use of particle accelerators is discussed and put in the context of competitive technologies: electron diffraction and microscopy and laser-based photon science. Advantages and disadvantages of various technologies are explored. Requirements for new x-ray sources that will drive accelerator research and development are discussed, focusing on the big challenges ahead.