  We describe aspects of the role that diquark correlations play in
understanding baryon structure and interactions. The significance of diquarks
in that application motivates a study of the possibility that dense hadronic
matter may exhibit diquark condensation; i.e., quark-quark pairing promoted by
a quark chemical potential. A Gorkov-Nambu-like gap equation is introduced for
QCD and analysed for 2-colour QCD (QC2D) and, in two qualitatively different
truncations, for QCD itself. Among other interesting features, we illustrate
that QC2D with massive fermions undergoes a second-order transition to a
superfluid phase when the chemical potential exceeds 0.5 m.pi. In the QCD
application we illustrate that the sigma= <bar-q q> .neq. 0 phase, which
determines the properties of the mass spectrum at zero temperature and chemical
potential, is unstable with respect to the superfluid phase when the chemical
potential exceeds approx. 2 sigma, and that at this point the diquark gap is
large, approx. 0.5 sigma. The superfluid phase survives to temperatures greater
than that expected in the core of compact stars.
