This thesis compares the Foreign Sovereign Immunities Act (FSIA) and the UN Convention on Jurisdictional Immunities of States and their Property as representative examples of current regimes of sovereign immunity, to determine what types of rules (whether ratione personae or ratione materiae) provide satisfactory results in cases involving sovereign wealth funds (SWFs). The adequacy of outcomes is assessed by establishing whether the ‘legitimate expectations’ of the SWFs’ contracting partners are preserved. Accordingly, it is discussed how the FSIA and the UN Convention rules on immunity from both jurisdiction and execution apply to all types of SWFs, whether they have a separate legal personality from the State or not. In each case, I examine the applicability of the ‘commercial activity’ exception. A recurring issue in practice involves the question whether an activity should be considered either de iure gestionis or de iure imperii when the ‘commercial activity’ exception is invoked. In light of the inherent difficulties of the traditional nature/purpose test, certain alternative means of discernment are suggested: an exercise of elimination; the use of the ‘purpose’ of SWFs’ specific transactions as opposed to their overall ‘purpose’; the use of ‘context’ and the method of ‘individuation’. Some sovereign immunity issues involving SWFs may be addressed through current rules on topic. Yet, those rules do not provide fully satisfactory results. Therefore, certain lex ferendae rules of sovereign immunity specific to SWFs are proposed.