Health is a universal human aspiration and a basic human need. The development of society, rich or poor, can be judged by the quality of its population's health, how fairly health is distributed across the social spectrum, and the degree of protection provided from disadvantage due to ill-health. Health equity is central to this premise and to the work of the Commission on Social Determinants of Health. Strengthening health equity--globally and within countries--means going beyond contemporary concentration on the immediate causes of disease. More than any other global health endeavour, the Commission focuses on the "causes of the causes"--the fundamental structures of social hierarchy and the socially determined conditions these create in which people grow, live, work, and age. The time for action is now, not just because better health makes economic sense, but because it is right and just. The outcry against inequity has been intensifying for many years from country to country around the world. These cries are forming a global movement. The Commission on Social Determinants of Health places action to ensure fair health at the head and the heart of that movement.