This lecture covers the principles of QM/MM simulations, focusing on the dynamical simulation of QMMM systems, the selection and protocol for QM/MM regions, and the workaround for the electron spill-out problem. It also discusses the application of QM/MM simulations to enzyme reactions, such as the lysozyme reaction mechanism and the ATPase reaction. The lecture explains the auxiliary bounding potential, the structure of ribozymes, and the theoretical studies of enzymatic reactions. Additionally, it introduces the CPMD code and its usage in QM/MM simulations.