Most tunnel barriers contain localized electronic statesnl(&Dgr;x, &egr;) in large amounts decreasing with distance &Dgr;xfrom the metal. The localized states hybridize with conduction electrons forming interface states with a decay width &Dgr;l∝exp(−2&Dgr;x&kgr;) and a correlation energy &Dgr;U* ∝ 1/&egr;r&Dgr;x. For &Dgr;U*>&Dgr;lthese states are localized, which yields a strong coupling to surface plasmons, phonons, and spins. These states cause diffuse surface scattering and enhance exponentially [∝ &Dgr;−1l ∝exp(+2&Dgr;x&kgr;)] the tunnel matrix element by resonant tunneling jRas compared to tunnelingj&fgr;¯through the whole potential barrier &fgr;¯. Consequently at voltages ‖eU‖ <&fgr;¯,jR(U,T) is identified by its strongerUandTdependencies and can even dominate overj&fgr;¯. The enhanced interaction of the localized electrons with surface plasmons, phonons, and spins yield strongU,T, and time dependencies in the tunnel current which produce giant zero‐bias anomaly and spin‐flip zero‐bias anomaly; capacitance changes; inelastic processes, noise, and barrier reduction with increasing temperature; and pair weakening, leakage current, and reduction of the Josephson current.