This paper investigates how and why test-based accountability (TBA), a global model for education reform, began to dominate educational debates in Norway in the early 2000s, and how this policy has been operationalised and institutionalised over time. In examining the adoption and retention of TBA in Norway, we build on the cultural political economy framework, in combination with a political sociology-driven approach to policy instruments. The analysis draws on two data sources: four White Papers and 37 in-depth interviews with top-level politicians, policy-makers and stakeholders, conducted between September 2017 and February 2018. The findings indicate that 'scandalisation' of Norway's below-expected PISA results and promotion of standardised testing as a neutral device contributed to the relatively abrupt adoption of national testing in the early 2000s. The increasingly dominant policy discourse equalising education quality and learning outcomes led to the institutionalisation of TBA, developed to ensure equity and quality standards in a decentralised education system. Increased visibility, benchmarking and administrative control are identified as key mechanisms in putting pressure on local actors to re-orient their behaviour. The study provides original insights into the drivers, expectations and strategies underlying TBA in a social democratic institutional regime.