This chapter describes the rapidly-developing area of σ- and π-bonded organo-lanthanide and -actinide chemistry. It deals with complexes containing p-bonded ligands, mainly cyclopentadienyls and substituted derivatives. The chapter examines the synthesis, structures and reactivity of Ln(III) complexes of the type Cp3Ln and Cp2LnX, as well as some Ln(II) complexes Cp2Ln. Schumann, Meese-Marktscheffel and Esser have reviewed lanthanide (III) chemistry with cyclopentadienyl and other π-donor ligands. The accessibility of more than one oxidation state for the actinides further complicates the preparation of homoleptic σ-bonded organometallic complexes of these elements. The great majority of well-characterized homoleptic σ-bonded organolanthanides are in oxidation state 3. The overwhelming majority of organolanthanide chemistry has been developed in systems with Cp as supporting ligands. The bis (cyclopentadienyl) lanthanide halides are crucial intermediates in this area, and a great deal of effort has been devoted to their synthesis and structural characterization.