Scholars in various fields have convincingly theorised instrument constituencies as a third type of policy subsystem alongside advocacy coalitions and epistemic communities. Instead of interest-based bargaining or shared causal beliefs surrounding a particular policy problem, these networks of actors organise around a common commitment to develop, maintain and expand particular policy instruments and instrumental models. This joint venture also underscores knowledge production and the use of evidence to influence policy-making. The paper applies the concept of instrument constiuencies in European education policy to assess its explanatory potential in an area that has seen a proliferation of expert and stakeholder networks in recent years. The comparative case study features two expert communities: the E4 group, an alliance of European stakeholders in higher education; and the Standing Group on Indicators and Benchmarks (SGIB), an informal expert group which advises the European Commission on EU-level targets and indicators in education. Established in the mid-2000s, both groups have been actively involved in shaping policy instruments: the E4 group as the author of the Standards and Guidelines for Quality Assurance in the European Higher Education Area, endorsed by higher education ministers of the Bologna Process; while the SGIB brings together national and international experts to advise on the measurement of the performance of national education systems in the EU. The paper examines, on the one hand, the institutionalisation dynamics and the different modes of knowledge generation which distinguish these groups and their modes of operation from other types of actors. On the other hand, the study assesses their potential for innovation in evidence-based policy-making, by contrasting the two instrumental models (bottom-up and top-down) of education governance represented by the two cases.