  I review the basic idea of $k_{\perp}$-factorization and its relation to
collinear factorization. Theoretical results in resummed perturbation theory
are summarized and the example of the heavy-flavour structure functions is
explicitly considered. Using these results one can investigate the small-$x$
behaviour of quantities that are independent of the non-perturbative parton
densities. In particular, one can introduce physical anomalous dimensions that
relate the scaling violations in different hadronic observables.
