International comparisons of health systems are widely used as a method for cross country learning, strategy development at the national level, and to demonstrate accountability. Comparisons may comprise entire health systems or subsystems within these, and may include many or few health systems. This paper describes strengths and weaknesses of different types of comparisons and methodological challenges involved in comparing different health systems. Finally, the paper emphasizes that caution must be exercised when transferring ideas from one system to another.