This article investigates the use of evidence in educational policy and politics, and how this use has changed over time. Using an analytical framework that combines research approaches from both political and educational science, evidence-related arguments in two major school reforms in the canton of Zurich, Switzerland are described. In conclusion, three hypotheses are presented concerning the question of how evidence evolved as a source of power in educational policy. The authors regard evidence as performing the function of both configuration and legitimisation. In addition, evidence seems to have become increasingly governmental since the 1940s. The use of evidence in the two school reforms under study can therefore be seen as an early antecedent of what sociologists of law would later call "de-parliamentarisation".