  The duality between partonic and hadronic descriptions of physical phenomena
is one of the most remarkable features of strong interaction physics. A classic
example of this is in electron-nucleon scattering, in which low-energy cross
sections, when averaged over appropriate energy intervals, are found to exhibit
the scaling behavior expected from perturbative QCD. We present a comprehensive
review of data on structure functions in the resonance region, from which the
global and local aspects of duality are quantified, including its flavor, spin
and nuclear medium dependence. To interpret the experimental findings, we
discuss various theoretical approaches which have been developed to understand
the microscopic origins of quark-hadron duality in QCD. Examples from other
reactions are used to place duality in a broader context, and future
experimental and theoretical challenges are identified.
