Quantum mechanics / molecular mechanics (QM/MM) is a type of embedding method that has been proven useful to describe reactivity in complex media such as enzymatic catalysis in biomacromolecules or reactivity of molecules in solution. In general, the QM subsystem is treated by a quantum mechanical method explicitly taking into account the electrons, while the rest of the system is treated at the MM level generating an interaction potential with the QM fragment. Many formulations of QM/MM exist in the literature, depending on the employed QM, MM, and the definition of their mutual interaction. The typical types of QM/MM embedding are classified as mechanical (no mutual polarization), electrostatic (QM polarization only), and polarizable (mutual polarization) embedding. Increasing the polarization might improve the description of electrostatic interactions between the QM and MM subsystems and it has been shown important to describe solvatochromic shifts for example, but the computational cost is also increased. Many varied strategies have been devised in the literature to accelerate QM/MM molecular dynamics (MD) simulations, either from efficient GPU implementations, averaging the MM interactions, using machine learning, or by combining multi-step integration with semi-empirical Hamiltonians, to mention just a few.