Under extreme compression, elements frequently undergo a profound reorganization of their electronic structure in order to accommodate Pauli exclusion, Coulomb repulsion, and core orthogonality constraints. This behaviour is particularly pronounced in alkali and alkaline earth metals, as well as in several other elemental systems, and can lead to the formation of high-pressure electrides (HPEs). In these phases, valence electrons localize within interstitial regions of the crystal lattice. Developing intuitive yet physically rigorous models of pressure-induced electronic redistribution remains a significant challenge. Combining accurate first-principles calculations with chemically intuitive bonding concepts makes it possible to rationalize and predict this behaviour. The structural chemistry and electronic topology of electrides will be examined using density functional theory, with particular attention to pressure-induced electronic transitions, orbital hybridization, and multicentre bonding. Quantum crystallographic approaches and optical observations will be discussed as probes of interstitial charge localization. Finally, crystal structure prediction guided by electron-density topology will be presented as a route to discovering new electrides at ambient and high pressure.