Scattering polarization in complex atoms poses interesting theoretical problems and is interesting for diagnostic plasma purposes. Here, an elementary theory on the generation and transfer of scattering polarization and Hanle effect in complex atoms (chiefly, transition metals and rare earths) is introduced. It is based on the quantum mechanical formulation of polarization transfer. The exposition is self-contained and can be (and has been) applied to treat very general problems. Numerical methods of solution are reviewed. I discuss non-trivial problems arising not from the formalism itself, but from its application to systems (complex atoms) with intrinsic spectroscopic peculiarities. In particular, they have a large number of relevant atomic levels massively interconnected between them by a huge amount of (resonant or quasi-resonant) spectral lines. This gives rise to surprising effects such as depolarization by interlocking, level over-polarization, and special Hanle effect regimes.