  We review the binding and energy level alignment of $\pi$-conjugated systems
on metals, a field which during the last two decades has seen tremendous
progress both in terms of experimental characterization as well as in the depth
of theoretical understanding. Precise measurements of vertical adsorption
distances and the electronic structure together with \textit{ab-initio}
calculations have shown that most of the molecular systems have to be
considered as intermediate cases between weak physisorption and strong
chemisorption. In this regime, the subtle interplay of different effects such
as covalent bonding, charge transfer, electrostatic and van der Waals
interactions yields a complex situation with different adsorption mechanisms.
In order to establish a better understanding of the binding and the electronic
level alignment of $\pi$-conjugated molecules on metals, we provide an
up-to-date overview of the literature, explain the fundamental concepts as well
as the experimental techniques and discuss typical case studies. Thereby, we
relate the geometric with the electronic structure in a consistent picture and
over the entire range from weak to strong coupling.
