A review. First-principles mol. dynamics (Car-Parrinello) simulations based on d. functional theory have emerged as a powerful tool for the study of phys., chem. and biol. systems. At present, using parallel computers, systems of a few hundreds of atoms can be routinely investigated. By extending this method to a mixed quantum mech. - mol. mech. (QM/MM) hybrid scheme, the system size can be enlarged further. Such an approach is esp. attractive for the in situ investigation of chem. reactions that occur in a complex and heterogeneous environment. Here, we review some recent applications of hybrid Car-Parrinello simulations of chem. and biol. systems as illustrative examples of the current potential and limitations of this promising novel technique. [on SciFinder (R)]