The year 2012 marked the golden anniversary of the Judd‐Ofelt (J‐O) theory as a tool used widely for the theoretical description of the spectroscopic properties of the lanthanide materials; in fact it is the only tool, as pointed out in the title of this chapter, that is used in ab initio type calculations. The chapter discusses the advantages and limitations of the Judd‐Ofelt theory when applied in ab initio and also semi‐empirical calculations. In addition to the standard non‐relativistic approach, the relativistic version is also presented to enable a theoretical description of those transitions that are observed in various experiments but forbidden within the original Judd‐Ofelt parameterization scheme of f‐spectra. Finally, an extensive list of important literature is provided to help the potential reader to understand the challenges of the theoretical description of the lanthanide ions, and most of all, to facilitate the application of existing theoretical models.