Publisher Summary Gas proportional detectors are well suited for the accurate measurement of X-ray diffraction data. The high signal-to-noise ratio and low background achieved by these systems make them particularly suitable for collecting quality data at high resolution. New developments are taking place to improve their counting rate capability, which currently limits their use for very demanding synchrotron radiation (SR) experiments. As X-ray counters, gas proportional detectors provide unrivaled dynamic range and sensitivity for photons in the 5–13 keV range. This makes them attractive for the acquisition of diffraction data in macromolecular crystallography. Initial developments were on multiwire proportional chambers (MWPCs), which are widely used as X-ray area detectors with laboratory X-ray sources. The advent of dedicated SR sources has led to the design of new generations of gas detectors. Gas proportional detectors use as a first step the absorption of an X-ray photon in a gas mixture high in xenon or argon.