CU-TP-863

[A.H. Mueller]{} [Department of Physics, Columbia University, New York, N.Y. 10027]{}

[**Abstract**]{}  A selected review of topics at the border of hard and soft physics is given. Particular emphasis is placed on diffraction dissociation at Fermilab and HERA. Recently, significant differences between diffraction dissociation at HERA and at Fermilab have become apparent. This may suggest that one already is reaching nonlinear (unitarity) effects which are extending from the soft physics region into the semihard regime of QCD.

Introduction
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The focus in this paper is on the regime of hardness near the borderline between hard and soft high energy collisions with a special emphasis on searching for nonlinear QCD effects. This is an opportune time for such a discussion as there is now a significant body of complementary data from deep inelastic scattering and from hadron-hadron collisions. Perhaps the major object of this review is to compare and contrast hadronic and deep inelastic collisions, especially diffraction dissociation where there are major differences between the hadronic and virtual photon initiated processes.

Sec.2 is devoted to a brief review of some soft physics results on total cross sections and diffraction dissociation. Despite the fact that total cross sections grow\[1\] as $(s/s_0)^\epsilon,$ 