This paper provides a theoretical definition of the institutionalization of assessment of public policies and undertakes an empirical comparison of the extent to which eighteen democracies have implemented it. Three categories of countries have thus been identified (pioneers, intermediates, and late developers), and six hypotheses are discussed in order to interpret the reasons for these marked differences between countries. The explanatory factors identified involve the practice of New Public Management, the level of budget deficits, external pressures (particularly from the European Union), the type of majority or consensus in government and the federalist or unitary structure of the democratic system, and finally, the independence of the Court of Auditors. Although each of these hypotheses seems to offer a plausible explanation for the assessment context in a given country, it must be conceded that they do not withstand statistical and qualitative-comparative analysis for the eighteen democracies analyzed here. In conclusion, an in-depth study of the historic processes of institutionalization and the reformative role of the public sector appears to be a necessary line of continued research.