Double-hybrid density functional (DHDF) theory (for reviews see ) represents a special case of fifth-rung functionals on 'Jacob's Ladder' (the fifth rung is where dependence on unoccupied orbitals enters). As such, DHDF theory resides on the seamline between DFT and wavefunction approaches. DHDFs are the most accurate DFT methods available to date for main group energetics, transition metal catalysis, electronic excitation spectroscopy, external magnetic and electric field induced properties, and vibrational frequencies. Its reliance on a second-order perturbation theory term for part of the correlation energy creates an Achilles' Heel for molecules with small band gaps, due to the presence of orbital energy differences in the denominator. Using the spin-orbital notation, the MP2-like Görling-Levy nonlocal correlation term has the following form: