While Ontario has received international accolades for its enactment of province-wide standardised testing upon the formation of the Education Quality and Accountability Office (EQAO), a closer look at provincial assessments over a 20-year span reveals successes as well as systemic tensions and fissures. The purpose of this paper is twofold. First, it offers a socio-historical account of EQAO policy and programme enactment. Second, it offers insight into the less-publicised politics of enactment within systems with respect to principles and methods of accountability and their link to instructional systems. Data and findings from various sources shed light on the realities of struggles, tensions in macro-political consensus and dissensus among various stakeholders, and the micro-politics at the school and district levels.