The latest advances in N-heterocyclic carbene-based organometallic chemistry have drawn increasing attention to this class of ancillary ligands as an attractive alternative to tertiary phosphines. Studies focusing on the fundamental steric and electronic factors characterizing this family of compounds are therefore essential for the rationalization of the activity observed for such organometallic complexes in metal-mediated organic transformations. This knowledge is also of fundamental importance in catalyst design efforts. This review intends to provide a comprehensive overview of the progress in this area.