  The hadronic final state in electron-proton collisions at HERA has provided a
rich testing ground for development of the theory of the strong force, QCD. In
this review, over 200 publications from the H1 and ZEUS Collaborations are
summarised. Short distance physics, the measurement of processes at high energy
scales, has provided rigorous tests of perturbative QCD and constrained the
structure of the proton as well as allowing precise measurements of the strong
coupling constant to be made. Non-perturbative or low energy processes have
also been investigated and results on hadronisation interpreted together with
those from other experiments. Searches for exotic QCD objects, such as
pentaquarks, glueballs and instantons have been performed. The subject of
diffraction has been re-invigorated through its precise measurement, such that
it can now be described by perturbative QCD. After discussion of HERA, the H1
and ZEUS detectors and the techniques used to reconstruct differing hadronic
final states, the above subject areas are elaborated. The major achievements
are then condensed further in a final section summarising what has been
learned.
