As one of the most important chemical reactions, S_N2 reactions play a central role in both synthetic chemistry and the development of mechanistic paradigms. Most of the previous attention has centered on substitution at carbon, whereas displacement at nitrogen is clearly less studied and less understood. The S_N2@N reactions have enormous synthetic potential, especially in heterocyclic chemistry, which is a cornerstone of medicinal chemistry, and has been receiving increasing attention, both experimentally and computationally.