The theoretical modeling of lanthanide and actinide complexes in condensed media is challenging for theoretical chemistry, due in part to the intrinsic general challenge of devising a reliable model of the species in such environments and the necessity of ‘chemical’ accuracy. This chapter discusses the requirements, challenges, and pitfalls associated with attempting to theoretically model molecular properties for such systems that can be directly compared to experimental data, such as valence excitation spectra, core‐excitation spectra, thermodynamics of chemical reactions, and redox properties. Spin‐orbit effects are in general important to account for in excited‐states calculations and in chemical reactions involving trans‐uranium actinides and in many cases also uranium complexes. The most rigorous framework to include spin‐orbit interaction is to treat it a priori in a four‐ or two‐component relativistic framework. The Ultraviolet–visible (UV‐Vis) spectroscopy of lanthanide and actinide elements directly reflects the electronic structure of the species involved.