This article investigates the impact of the Third Evaluation Round of the Group of States against Corruption (GRECO) on party finance laws in eighteen Northern, Western and Southern European states. It particularly focuses on the transparency stipulations regarding political parties’ annual accounts. Five indicators are analyzed: (1) the obligation of parties to submit their accounts, (2) the availability of a standardized form, (3) the obligation to make the accounts public, (4) the independence of the auditor, and (5) the independence of the monitoring body. On the basis of this analysis, a global transparency score is calculated, allowing us to assess the impact of Greco. Our study shows that Greco had a significant impact: the legal framework of a large number of countries has been amended, in the sense that the level of transparency regarding the parties’ accounts has considerably improved.