Quantum embedding allows one to divide a bulk problem into sub-problems such that a particular region of interest, known as the "active region" or "impurity", is treated with a more accurate and thus more expensive method, while the rest of the system is described using a mean-field approach. There are many different versions of quantum embedding techniques, which are classified based on the level of theory used to analyze the sub problems and the key quantum variable used to connect the high-and low-level methods.