Measured in terms of increases in average expectation of life country-by-country, the large majority of the world's population benefited from major improvements in health in the second part of the twentieth century. Notable exceptions to this favorable trend are most of the countries of sub-Saharan Africa and countries of the former Soviet Union. But the country averages conceal persistent and significant differences according to social status. The aim of the work of the Commission on Social Determinants of Health (CSDH) of the World Health Organization is to study these differences and to make recommendations for corrective action. Set up in March 2005, the 19-member Commission, chaired by Michael Marmot of University College London, published an Interim Report, titled “Achieving Health Equity: From Root Causes to Fair Outcomes,” in September 2007. An excerpt from this 61-page report—the section on Health Inequality, Inequity, and Social Determinants of Health—is reproduced below. It presents a crisp statistical description of characteristic features of social differentials in health status commonly found in all countries, including those exhibiting the most favorable average expectancies of life. The presentation draws on emerging work on this topic, extensively cited in the report. The full text of the Interim Report can be accessed through CSDH's web site: «http://www.who.int/social_determinants/en».