MotivationElaborated in their current form in Busan in 2011, and reiterated in Geneva in 2022, the four Principles of Effective Development Co‐operation comprise country ownership, focus on results, inclusive partnerships, and transparency and mutual accountability. Framed to guide more effective development assistance, their measurement and impact has not been systematically studied.PurposeWe ask, what do we know about adherence to the principles and better development outcomes? What can we learn about this relationship using the Global Partnership for Effective Development Co‐operation (GPEDC)'s monitoring framework?Methods and approachWe define and measure adherence to the four principles using the GPEDC framework and indicators. We then explore the association between these indicators and development outcomes (for example, economic growth and poverty) using correlation, descriptive analysis, and analysis of data quality.FindingsAcross countries, the empirical relation between the currently available GPEDC data and development outcomes is tenuous as best. Shortcomings in the data explain much of the lack of evidence. Some shortcomings could be fixed straightforwardly with adjustments to the indicators and data collection, but many relate to inherent challenges to measurement of the four principles.Policy implicationsMore precise definition of indicators of adherence to the principles, with wider coverage of countries, and with annual measurement would help —not necessarily for their instrumental value, but as much, if not more, to raise the profile of the principles themselves. The universally agreed objectives of strengthening international partnerships and cooperation, as well as of building inclusive, effective, accountable, and transparent institutions, as declared in the 2030 Agenda for Sustainable Development, are goals well worth pursuing.