$k_{\perp}$-Factorization and Perturbative Invariants at Small $x$

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.