This Element introduces the concept of institutional weakness, arguing that weakness or strength is a function of the extent to which an institution actually matters to social, economic or political outcomes. It then presents a typology of three forms of institutional weakness: insignificance, in which rules are complied with but do not affect the way actors behave; non-compliance, in which state elites either choose not to enforce the rules or fail to gain societal cooperation with them; and instability, in which the rules are changed at an unusually high rate. The Element then examines the sources of institutional weakness.