Since the mid 1990s, N-heterocyclic carbenes (NHCs) have proved to be a versatile class of ancillary ligands in catalysis. The NHC chemistry of Group 9 metals (Co, Rh, Ir) is one of the most developed areas in this field and is the subject of the present Chapter. This section covers the most relevant catalytic applications, along with stoichiometric model reactions, except for catalytic oxidation and reduction reactions which are covered in Chapters 12 and 13 respectively.