The theoretical modelling of chemically active transition metal (TM) centres is a notoriously difficult task. The metal-ligand interactions in these complexes are often highly directional and the concoction of suitable analytic interaction potentials can be far from trivial. The situation is rendered even more difficult by the fact that at finite temperature, the system might switch dynamically between different bonding situations or exhibit several energetically close-lying spin states which are all characterized by different coordination numbers and geometries. In this article, we describe the structural, dynamical and reactive properties of complex TM-containing systems with the help of a mixed quantum mechanical/molecular mechanical (QM/MM) molecular dynamics approach, in which the TM centre is described with generalized gradient corrected density functional theory embedded in a classical force field description. The power of such a combined Car-Parrinello/molecular mechanics approach is illustrated with a number of representative examples ranging from enantioselective TM catalysts to radiopharmaceuticals and metalloenzymes.