N-heterocyclic carbene (NHC) is mainly used as a ligand for transition metals. Strategic development for the construction of chemical structures of NHC gave success due to stabilize for unstable carben by delocalization of two adjacent nitrogen atoms, the chemistry using NHC blossomed greatly. These are also called to be Wanzlick-Arduengo carbons. At present, it is also utilized as a ligand of an olefin metathesis catalyst, and the possibility of the catalyst is drastically expanded.