<p>Electron localization and delocalization are fundamental concepts for interpreting chemical bonding, aromaticity, reactivity, spectroscopy, and the electronic properties of molecules and materials. This Guest Editorial introduces the theoretical framework underlying the Special Collection “<i>Electron Localization and Delocalization: From Fundamentals to Applications</i>” in <i>Structural Chemistry</i>. The many-electron wavefunction, the one-electron density, the pair density, and the exchange–correlation density are briefly discussed first, followed by the definitions of the localization and delocalization indices (LIs and DIs) used to quantify electron confinement within atomic basins and electron sharing between atoms in a molecule. The interdependence of these indices through the Bader summation rule is emphasized along with their casting into electron Localization–Delocalization Matrices (LDMs). The editorial also outlines the effect of electron correlation on these descriptors and highlights the breadth of topics represented in the collection, including conceptual and methodological advances, bonding and aromaticity, density-derived reactivity descriptors, electronic force fields, molecular design for optical and electronic function, crystalline and materials applications, correlated-electron and superconducting phenomena, and biologically relevant systems.</p>