  Deep-inelastic scattering at small x is a new field of QCD phenomenology
which HERA experiments started to explore. The data show the feature expected
by the perturbative Regge asymptotics. We describe the parton picture of
structure function evolution both for increasing $Q^2$ and for decreasing $x$
and discuss the leading logarithmic approximation of QCD on which this picture
is based. The steep rise of the parton density towards smaller $x$ gets
saturated by parton recombination. An essential improvement of the leading
logarithmic approximation has to be worked out in order to satisfy the
unitarity conditions and in this way to describe the parton recombination in
perturbative QCD. We review some ideas of the perturbative Regge asymptotics
and some recent theoretical results about multi-reggeon exchange in QCD. Talk
at the Tennessee Intern. Symposium on Rad. Corrections, prepared for the
proceedings.
