This article presents a quantitative measurement of the political legitimacy of states in the late 1990s and early 2000s for 72 states containing 5.1 billion people, or 83 per cent of the world's population. First, the concept of state legitimacy is defined and justified. The definition includes the subjects, objects and sub-types of legitimacy. A strategy to achieve replicable cross-national measurements of legitimacy is then outlined and implemented, including a discussion of data sources and three alternative aggregation methods. The results are briefly examined and tested, and the uncertainties of quantitative measures discussed. Finally, the role of supplementary qualitative measurement is considered.