In high centre-of-mass energy lepton-nucleon collisions the space-time time
resolution of partonic processes can be {\it fine-tuned} within a dynamical
range which is unattainable in hadronic collisions. Replacing nucleons by
nuclei of variable atomic number enables one to tune the strength of colour
forces. The experimental program of high energy electron-nucleon and its
extension to electron-nucleus collisions should thus give an unique opportunity
to experimentally explore the transition between the soft and hard interactions
of small and extended partonic systems. Such an experimental program, which can
be realized at DESY and/or BNL with relatively modest cost, is discussed in
this talk.Comment: Plenary talk at the PANIC conference, Uppsala, June 1999. 8 pages. 3
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