The multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) theory is a rigorous and powerful method to simulate quantum dynamics in complex many-body problems. This approach extends the regular MCTDH theory of Meyer, Manthe, and Cederbaum to include several dynamically contracted layers whose equations of motion are determined from variational principle. In this talk I will discuss the general derivation of the theory, the scaling of the method, and the application of the theory to simulate dynamics of electron transfer reactions in the condensed phase. Furthermore, a new generalization of the theory, the ML-MCTDH theory with second quantization (ML-MCTDH/SQ) will be presented to treat many-body identical particle (fermion or boson) systems.