nternational comparisons of health system performance exert major influence on the public and on policy makers. These comparisons allow for reflection on and evaluation of national performance, provide empirical bases to drive reform, and serve as a way to promote accountability. Most international comparisons seek to identify high performers, often conceptualized as health systems with the best outcomes or most value for money. Even though these notions seem relatively straightforward, operationalizing them is difficult. There are at least 3 key challenges of conducting international comparisons: drawing the boundaries of the health system, managing limitations of data, and accounting for different values inherent in national systems. Without understanding and addressing these challenges, cross-national comparisons will fail to improve health policy and may lead to misinterpretations and poor policy making.