Lanthanide based materials are used in everything from phosphors for light bulbs and LEDs, to scintillators for medical imaging purposes to magnets. They also show potential for spintronics and for increasing the efficiency of solar cells. Knowing the energy of the lanthanide 4f and 5d states is important for understanding the electronic structure and thus the optical properties of lanthanide based materials. In this thesis it is shown that an empirical model used to predict the electronic structure of lanthanide doped compounds can be used to predict the electronic structures of fully concentrated lanthanide based compounds. An example is then given that shows experimentally the systematic behaviour observed. It is also shown that it is possible to engineer the energy difference between the lanthanide 4f and 5d states in a lanthanide based compound. This may be done mechanically, by applying an external perturbation, such as by inducing stresses in an SmS thin film by polishing, or chemically, by alloying with a similar compound, in this case EuO with EuN, or by changing the stoichiometry of the compound, as in the case of Sm rich SmS.