In this Chapter, the catalytic applications of organometallic species â either pre-formed or generated in situâ based on Group 8 transition metals and N-heterocyclic carbene (NHC) ligands are surveyed. Thus far, only a few reports on the use of NHCâ Fe complexes in organic catalysis are available, although significant work has been reported in the related field of biocatalysis. Contrastingly, the chemistry of NHCâ Ru complexes has reached an unprecedented level of maturity, thanks to the relentless research efforts thrown into the development of olefin metathesis catalysts. Other carbon skeletal transformations based on NHCâ Ru promoters include cyclopropanation, allylation, or cycloisomerisation reactions. Lastly, with only two reports to date concerning olefin metathesis and transfer hydrogenation, NHCâ Os-based catalysis can hardly be considered anything else than a curiosity.