  While the main features of atomic nuclei are well described by nuclear
mean-field models, there is a large and growing body of evidence which
indicates an important additional role played by spatially-correlated
nucleon-nucleon structures. The role of nucleonic structures was first
suggested by Heidmann in 1950 to explain pick-up reactions of energetic
nucleons. Since then a steady flux of new experimental evidence has confirmed
the presence of similar structures inside atomic nuclei, dominated by
correlations between pairs of nucleons. The role of these internal
nucleon-nucleon correlations has been established using various energetic
probes like photons, pions, leptons and hadrons. These correlated structures
are essential for understanding the interaction of particles with nuclei and
their presence provides an explanation of many specific nuclear phenomena
including backscattered protons, copious deuteron production, sub-threshold
particle production, neutrino interactions with nuclei and the EMC effect. On
the theoretical side, these measurements have stimulated a large number of
phenomenological models specifically devised to address these enigmatic
observations. While reviews exist for specific interactions, there is currently
no published commentary which systematically encompasses the wide range of
experimental signatures and theoretical frameworks developed thus far. The
present review draws together the synergies between a wide range of different
experimental and theoretical studies, summarises progress in this area and
highlights outstanding issues for further study.
