  Jet production in electron-proton collisions at HERA provides a unique
testing ground for Quantum Chromodynamics (QCD). Apart from the determination
of the strong coupling constant $\alpha_s$, ep jet data may especially be used
to gain insight into the dynamics of the exchanged parton cascade, whose
structure is probed by the high-$E_T$ dijet system; thus information on the
parton content of the proton and (quasi-)-real and virtual photons is obtained.
This report touches some of these aspects revealed in recent jet data from the
HERA experiments which are testing perturbative QCD at the limits of
applicability.
