  Quantum Chromodynamics (QCD) is the theory governing the strong interaction
of particles. It describes the interactions that bind quarks and gluons into
protons and neutrons, and binds these into nuclei. We believe QCD to be as
fundamental and complete as QED, the theory of electromagnetic interactions,
whose predictions have been tested to more than ten decimal places. If it were
possible to make calculations in QCD the same way we can in QED, we would have
removed one of the biggest obstacles in the way of understanding matter in the
universe. Unfortunately, the properties of QCD make such calculations
impossible at present. Historically, there have been two approaches to this
problem. First, we work to improve our ability to solve QCD, with the most
visible effort being the field of Lattice QCD. Second, we make models of QCD
that attempt to incorporate what we believe to be the most important
symmetries, dynamics, or degrees of freedom, and then test these models against
experimental measurements sensitive to these assumptions. Even the earliest
quark models of hadrons structure and the simplest bag models have had great
success, far beyond any reasonable expectation, indicating that these models
have isolated some of the key features of QCD. More detailed models and ever
more sophisticated experimental tests are significantly improving such details,
and helping to better identify the most relevant features of QCD, one of the
key missing pieces in our understanding of the nature of matter.
