The Quantum Information Theory (IT) perspective on entropic origins of the molecular electronic structure and chemical reactivity is examined in this book. The classical IT of Fisher and Shannon has already provided sensitive probes of the chemical bond patterns in molecules, generating tools for the bond spatial localization, formulating entropic measures of its overall multiplicity, and providing indices of the bond covalent/ionic composition. However, the amount of information carried by the particle distribution alone is not sufficient for a complete IT description of complex wavefunctions in molecular quantum mechanics, e.g., degenerate electronic states. The latter exhibit finite spatial phases and generate nonvanishing electronic currents. This monograph emphasizes the entropic chemical concepts due to the phase part of electronic states.