First-principles molecular dynamics (Car-Parrinello) simulations based on density functional theory have emerged as a powerful too[ for the study of physical, chemical and biological 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 mechanical-molecular mechanical (QM/MM) hybrid scheme, the system size can be enlarged further. Such an approach is especially attractive for the in situ investigation of chemical reactions that occur in a complex and heterogeneous environment. Here, we review some recent applications of hybrid Car-Parrinello simulations of chemical and biological systems as illustrative examples of the current potential and limitations of this promising novel technique.