  The last decade has seen a marked shift in how the internal structure of
hadrons is understood. Modern experimental facilities, new theoretical
techniques for the continuum bound-state problem and progress with
lattice-regularised QCD have provided strong indications that soft quark+quark
(diquark) correlations play a crucial role in hadron physics. For example,
theory indicates that the appearance of such correlations is a necessary
consequence of dynamical chiral symmetry breaking, viz. a corollary of emergent
hadronic mass that is responsible for almost all visible mass in the universe;
experiment has uncovered signals for such correlations in the
flavour-separation of the proton's electromagnetic form factors; and
phenomenology suggests that diquark correlations might be critical to the
formation of exotic tetra- and penta-quark hadrons. A broad spectrum of such
information is evaluated herein, with a view to consolidating the facts and
therefrom moving toward a coherent, unified picture of hadron structure and the
role that diquark correlations might play.
