Why do some groups live longer and healthier lives than others? This lecture addresses this question by introducing key concepts and theoretical frameworks for understanding the social determinants of health and health inequalities. It discusses major forms of health inequality, with particular attention to socioeconomic and gender inequalities in health outcomes. Drawing on empirical evidence from contemporary China, the lecture illustrates how patterns of health inequalities are shaped by broader processes of social transformation. It then examines the mechanisms linking social transformation to changes in population health and health inequalities, and their implications for policies aimed at reducing health inequalities.