Perturbative QCD and infra-red-finite measures of the jet structure of hadronic final states

First order QCD perturbation theory is used to investigate the energy-momentum structure of hadronic final states, in both e+e- annihilation and leptoproduction, at high energies. A simple derivation of the Sterman-Weinberg formula (a perturbative approximation applied to infra-red-finite quantities) for the 2-jet fraction in e+e- is described. The leptoproduction analogue of this formula is derived and the phenomenology of jets in leptoproduction discussed, with particular emphasis on those features of the perturbative QCD results which differ from the expectations of the naive parton model with psub(T)-limited jets arising from the fragmentation of partons into hadrons. (U.K.)