The levels of impactful entrepreneurship generated in different economies depend on the quality of a range of supporting conditions, which can be analysed using the entrepreneurial ecosystem concept. This sees entrepreneurship as being embedded in a system involving various institutional conditions, resource endowments, interactions and feedbacks. To support policy makers diagnose the vitality of entrepreneurial ecosystems and identify policy priorities, the OECD has developed an Entrepreneurial Ecosystems Diagnostics report with comparative statistics on entrepreneurship levels and drivers for OECD countries. This companion paper presents details of the report’s methodological approach. It covers the conceptual framework, the choice of indicators, data normalisation, aggregation, missing value imputation, and sensitivity analyses and robustness tests with respect to alternative methodologies. The paper concludes by outlining the methodological limitations and directions for future development of the diagnostics.