Computational studies on the reactivity of transition metal complexes featuring N-heterocyclic carbene (NHC) ligands are reviewed. Reactions involving the direct participation of NHC ligands are first considered, including reductive elimination, migratory insertion and cyclometallation. Subsequent sections survey processes where NHC ligands act as spectator ligands, in particular those where their electronic and steric characteristics impact on reactivity. Small molecule activation is discussed, as well as the extensive studies on the behaviour of Grubbs' 2^nd generation metathesis catalyst and related species.