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Extreme states of matter with high temperatures and pressures can be created by irradiating matter with either intense X-rays emitted by X-ray free-electron lasers (FELs), and by heating and/or compression with optical lasers and then using the FEL X-rays as a probe. We provide here a very basic introduction to this burgeoning field, highlighting a few specific experiments, and signposting some directions for future exploration.