In this seminar, some basic concepts will be presented dealing with the electronic structure in a solid host of the ions having a 4fⁿ configuration (n=1-13). These ions belong to the series called “lanthanide” or “rare earth” ions. Secondly, the intraconfigurational optical transitions of the 4fⁿ-4fⁿ type will be briefly discussed. The possibility of using the 4f⁶ Eu³+ ion as a spectroscopic probe of the local structure around lanthanide ions in a given material will be proposed and discussed. Case studies relative to the luminescence spectroscopy of Eu3+ present as a dopant in the complex silicates Ca₃Sc₂Si3O₁₂ and Ca₃Y₂Si₃O₁₂ will be described and commented.