Over the years, scientists have proposed several "rules-of-thumb" to predict the behavior of matter under pressure, though these have been continuously revised with advancements in the field. Other researchers have attempted more quantitative approaches, calculating how atomic properties evolve with compression, such as developing pressure-dependent electronegativity scales and atomic radii, or investigating the activation of core-electrons through extreme oxidation via density functional theory (DFT) calculations. This constant refinement of the chemical rules at high-pressure conditions aims to explain and predict experimental observations that, more often than not, defy our expectations. In this review, we focus on one particular exotic phenomenon that challenges our comprehension of matter under extreme pressure: the formation of high-pressure electrides (HPEs). We navigate through historical works and theoretical models of HPEs, seeing how they have evolved dur-Figure : Schematic of phenomena that occur at pressures ranging from atmospheric conditions (1 atm ∼ 0.0001 GPa) to those that may be found in the interiors of gas-giant planets such as Saturn, or even stars such as brown dwarfs. Among the possible phenomena we show: the formation of diamond at pressures comparable to those found in the Earth's crust; the post-perovskite phase transition of MgSiO 3 , which plays a major role in the geophysical properties of the interior of our planet; 37 the metal-to-insulator transition of sodium upon transformation to the Na-hP 4 phase; 4 and the target used for inertial-confinement-fusion reactions at the National Ignition Facility (Lawrence Livermore National Laboratory). ing the years, ultimately showing that HPEs can be understood using the intuitive chemical picture of orbitals and bonds.