Guarantor institutions (such as electoral commissions and anti-corruption watchdogs, which supposedly comprise the so-called ‘fourth’ or ‘integrity’ branch of the state) are increasingly of interest to constitutional scholars. In a given political context, a guarantor institution is a tailor-made constitutional institution, vested with material as well as expressive capacities, whose function is to provide a credible and enduring guarantee to a specific non-self-enforcing constitutional norm (or any aspect thereof). Arguing that guarantor institutions are more trustee-like than agent-like in character, this chapter defends the claim that the design of any guarantor institutions should seek to ensure that it has: (i) sufficient expertise and capacity to perform its functions effectively; (ii) sufficient independence from political, economic, or social actors with an interest in frustrating the relevant norm it is meant to guarantee; and (iii) sufficient accountability to bodies with an interest in upholding the relevant norm.