Hybrid quantum mechanics/molecular mechanics (QM/MM) is a powerful tool in computational chemistry. It enables the investigation of intricate chemical properties and processes within complex systems and has been widely used in the study of biological problems, including enzymatic processes and drug design. Additionally, it has been used in various other fields such as heterogeneous catalysis and nanochemistry. QM/MM is unique in its multiscale nature, where higher-level accurate QM methods are applied to regions of primary interest while lower-level cost-effective MM methods are used for the surrounding environment, thereby minimizing the total computational expense. Although QM/MM is less accurate than pure QM methods and presents challenges such as the proper partitioning of QM and MM regions, managing QM/MM boundary treatments, and addressing overpolarization issues of QM electron densities near the MM region, it remains a preferred method for studying complex systems due to its balanced accuracy and computational efficiency.