This chapter provides a critical review of the use of 2‐ and 4‐component relativistic Hamiltonians combined with all‐electron methods and appropriate basis sets for the study of lanthanide and actinide chemistry. These approaches provide in principle the more rigorous treatment of the electronic structure but typically demand large computational resources due to the large basis sets that are required for accurate energetics. For density functional theory (DFT) calculations one needs to select the appropriate density functional approximation (DFA) on basis of assessments for lighter elements because little or no high‐precision experimental information on isolated molecules is available for the f elements. The chapter discusses the available relativistic Hamiltonians that form a prerequisite for the treatment of electronic structure of heavy elements. It focuses on methods suitable for an all‐electron treatment of f elements.