Many other compounds, proposed by theory to become HPEs are waiting to be accessed at pressures only accessible via dynamic compression. Notable among them are Al above 3.2 TPa characterized by a host-guest type structure; 59 fcc carbon, computed to be stable above ∼20 TPa; 61 and several allotropes of Na predicted to be preferred over the Na-hP 4 phase around 300 GPa and thousands of degrees Kelvin, 101 or at zero temperature and at pressures above 1.75 TPa. However, this survey highlights one crucial aspect in the research done so far on HPEs: compelling experimental proof for the interstitial electron localization at high-pressure is still missing. The physical properties of electrides and related systems at ambient conditions have been interrogated experimentally, often using spectroscopic probes, and the results interpreted in terms of localized electrons trapped in interstices. So far the interstitial charge localization in HPEs has not been probed either directly or indirectly via experiments, but instead inferred from the results of first principles calculations for structures that are either predicted to be stable or those whose X-ray diffraction (XRD) patterns are consistent with experiments. Advances in HP-diagnostic techniques are required to develop methods towards this end. For example, single crystal XRD performed in a DAC could possibly be used to quantify the excess electron density in the interstitial cavities, especially for highly symmetric systems. The enormous progress in nuclear magnetic resonance (NMR) spectroscopy at high-pressure could open new venues for the characterization of HPEs.