A review. The theor. modeling of chem. active transition metal (TM) centers 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 temp., the system might switch dynamically between different bonding situations or exhibit several energetically close-lying spin states which are all characterized by different coordination nos. and geometries. In this article, the authors describe the structural, dynamical and reactive properties of complex TM-contg. systems with the help of a mixed quantum mech./mol. mech. (QM/MM) mol. dynamics approach, in which the TM center is described with generalized gradient cor. d. functional theory embedded in a classical force field description. The power of such a combined Car-Parrinello/mol. mechanics approach is illustrated with a no. of representative examples ranging from enantioselective TM catalysts to radiopharmaceuticals and metalloenzymes. [on SciFinder (R)]