PNOs, initially formulated in the early days of quantum chemistry by Edmiston and Krauss as "pseudonatural orbitals," saw initial popularity in the PNO-CI and the IEPA-PNO approaches. However, the method dwindled in use due to the rise in popularity of direct configuration interaction (direct CI) approaches. With the rise of local correlation methods, Neese et al. realized that PNOs can form a natural basis to express the virtual space of a given LMO pair, and developed the LPNO-CEPA and LPNO-CCSD methods. Since truncated PNOs represent the most compact representation of the virtual space of an LMO pair, PNO-based local correlation methods are orders of magnitude more efficient than their PAO counterparts. In doing so, they introduced the next generation of local correlation methods by resurrecting PNOs from the annals of quantum chemistry. PNOs are defined as the eigenvectors of the pair density of a molecular orbital pair ij.