This chapter describes some important aspects of the shell structure, relativistic effects, and electron correlation effects for lanthanide and actinide atoms and molecules, mainly using the example of Cerium (Ce) and Thorium (Th). It turns to a discussion of the electronic structure of cerium‐bis(η8‐cyclooctatetraene), cerocene, which fascinated this author for more than two decades, as well as a related cerium (III) ‐based molecular Kondo system, i.e., bis(η8‐pentalene) cerium. These systems feature many problems of an accurate relativistic correlated description of their electronic structure and moreover leave plenty of room for alternative interpretations of their electronic ground state as well as the involvement of 4f orbitals in chemical bonding. In order to describe accurately the electronic structure of many‐electron systems, highly correlated approximate solutions of the Schrödinger equation built from the best possible relativistic Hamiltonian are needed.