Political actors derive influence both from their official position and from their own personal standing. However, political science has proven to be better equipped to study formal powers and institutions. The study of informal arrangements, where actors rely on personal connections and authority, is more challenging. This has arguably led to the predominance of an institutional focus in research. We argue for the study of informal sources of power as an equally important area of research. Drawing from historical and contemporary examples, we reintroduce the concepts of auctoritas and potestas to underline the difference between individual and institutional sources of influence. We discuss the various obstacles to measurement and outline attempts proposed to date in the literature.