This chapter focuses on two classes of anionic O-donors: alkoxides and aryloxides, and ß-diketonates. These classes of compound will illustrate important aspects of the chemistry of lanthanides and actinides. There are of course many other anionic O-donors which form complexes with the f-block elements. The chapter provides to homoleptic complexes with simple monodentate ligands. Similarly, siloxide complexes will be omitted. It deals with aromatic N-donors and Schiff base ligands. The chapter examines simple ‘Inorganic’ complexes with S and P donor ligands. In addition to the complexes described there are numerous organolanthanide and -actinide phosphido and thiolato complexes, most of which contain Cp or related ligands. The coordination geometry of lanthanide and actinide complexes is dictated almost exclusively by steric considerations: the ligands pack around the metal ions in such a way as to minimise interligand repulsions, and the coordination number is determined by the steric bulk of the ligands.