In this Chapter, the catalytic applications of organometallic species —either pre-formed or generated in situ— based on Group 8 transition metals and N-heterocyclic carbene (NHC) ligands are surveyed. Thus far, only a few reports on the use of NHC–Fe complexes in organic catalysis are available, although significant work has been reported in the related field of biocatalysis. Contrastingly, the chemistry of NHC–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 NHC–Ru promoters include cyclopropanation, allylation, or cycloisomerisation reactions. Lastly, with only two reports to date concerning olefin metathesis and transfer hydrogenation, NHC–Os-based catalysis can hardly be considered anything else than a curiosity.