During the last decade the field of computational chem. has experienced an enormous progress. Due to the exponential increase in computer power and the development of new computational methods it has now become possible to treat many complex chem. and biol. problems on an accurate and realistic level. Ab initio mol. dynamics simulations based on d. functional theory are among the most powerful of these new tools. Using parallel computers, systems of a few hundred of atoms can be studied routinely. By extending this approach to a mixed quantum-classical (QM/MM) hybrid scheme, the system size can be enlarged even further. This method paired with enhanced sampling techniques is esp. attractive for the in situ investigation of complex chem. and biochem. reactions that occur in a heterogeneous environment. The power of this approach will be demonstrated on different illustrative examples with a particular emphasis on applications to enzymic reactions and photoactive proteins. [on SciFinder (R)]