Shortly after the discovery of ferrocene, Reynolds and Wilkinson demonstrated that actinide elements are capable of forming organometallic compounds containing the cyclopentadienyl (Cp) ligand. The compound they reported, [Cp,UCI], was the first of a host of organoactinide complexes that contain Cp and substituted Cp ligands. In 1968, Streitwieser and Miiller-Westerhoffsynthesized uranocene, [(C,H,),U], a molecule whose existence had been predicted five years earlier by R. D . Fischer. Because uranocene has no analogue in transition-metal chemistry, its synthesis stimulated new interest in organoactinide chemistry and in comparisons with organotransition-metal chemistry. Since that time, Cp and substituted Cp ligands, such as pentamethylcyclopentadienyl (Cp *), have become the most important and flexible ligands in actinide organometallic chemistry. In the past 15 years especially, the bonding of Cp ligands to actinide elements has been extensively probed by both experiment and theory. There are both significant similarities and equally significant differences in the bonding of the Cp ligand to the f-element metals compared to the d-element metals.