International comparison of health system performance has become increasingly popular, made possible by the rapidly expanding availability of health data. It has become one of the most important levers for prompting health system reform. Yet, as the demand for transparency and accountability in healthcare increases, so too does the need to compare data from different health systems both accurately and meaningfully. This timely and authoritative book offers an important summary of the current developments in health system performance comparison. It summarises the current state of efforts to compare systems, and identifies and explores the practical and conceptual challenges that occur. It discusses data and methodological challenges, as well as broader issues such as the interface between evidence and practice. The book draws out the priorities for future work on performance comparison, in the development of data sources and measurement instruments, analytic methodology, and assessment of evidence on performance. It concludes by presenting the key lessons and future priorities, and in doing so offers a rich source of material for policy-makers, their analytic advisors, international agencies, academics and students of health systems.