Over the past four years, the OECD has conducted a series of in-depth reviews of the policies and institutions that underpin the measurement and improvement of health care quality in 15 different health systems. This synthesis report draws on key lessons from the OECD Health Care Quality Review series. The objective is to summarise the main challenges and good practices to support improvements in health care quality, and to help ensure that the substantial resources devoted to health are being used effectively in supporting people to live healthier lives. The overarching conclusion emerging across the Health Care Quality Review series concerns transparency. Governments should encourage, and where appropriate require, health systems and health care providers to be open about the effectiveness, safety and patient-centredness of care they provide. More measures of patient outcomes are needed (especially those reported by patients themselves), and these should underpin standards, guidelines, incentives and innovations in service delivery. Greater transparency can lead to optimisation of both quality and efficiency – twin objectives which reinforce, rather than subvert, each other. In practical terms, greater transparency and better performance can be supported by changes in where and how care is delivered; changes in the roles of patients and professionals; and employing tools such as data and incentives more effectively. Key actions in these three areas are set out in the twelve lessons presented in this synthesis report.