Accelerator-based hard X-ray sources (storage-ring synchrotron radiation, and X-ray free electron laser, or FEL) provide X-ray beams with high energy, high brilliance, short tens-of-picosecond-to-femtosecond pulses, and high coherence that are well suited for high-pressure studies. Developments in high-pressure technology, advanced X-ray optics and detectors, and synergies with theoretical computations have helped drive the rapid growth of high-pressure research using synchrotron and FEL X-rays. In this chapter, we present a brief review of the research field from a historical perspective, illustrated by selected aspects on research using the diamond anvil cell. We then highlight a few of the active areas in high-pressure X-ray research, including ultrahigh-pressure generation, amorphous materials at high pressure, phase transition kinetics, and materials metastability. Finally, an outlook on future directions and opportunities with the upgrades in both synchrotron and FEL facilities worldwide is presented.