Social epidemiology is a branch of epidemiology that focuses on the social distribution and social determinants of health. It emphasizes the inextricable link between social factors and health outcomes, suggesting that all epidemiological exposures are related to social conditions. The chapter introduces the field, highlighting its historical roots in the works of Chadwick and Durkheim, and its role in posing new research questions, developing new methods, and influencing government policy. The research questions in social epidemiology revolve around understanding the social determinants of health, which are the social and economic factors that influence health outcomes. These determinants include material, economic, and political factors, as well as life course events and social biology. Social inequalities in health are well-documented, with people from lower socioeconomic backgrounds experiencing worse health outcomes and lower life expectancies, even in wealthy countries. The chapter discusses the need to understand the pathways from social structure to health and the various attempts to identify these pathways, such as those outlined in the Commission on Social Determinants of Health (CSDH) report.Social epidemiology is a branch of epidemiology that focuses on the social distribution and social determinants of health. It emphasizes the inextricable link between social factors and health outcomes, suggesting that all epidemiological exposures are related to social conditions. The chapter introduces the field, highlighting its historical roots in the works of Chadwick and Durkheim, and its role in posing new research questions, developing new methods, and influencing government policy. The research questions in social epidemiology revolve around understanding the social determinants of health, which are the social and economic factors that influence health outcomes. These determinants include material, economic, and political factors, as well as life course events and social biology. Social inequalities in health are well-documented, with people from lower socioeconomic backgrounds experiencing worse health outcomes and lower life expectancies, even in wealthy countries. The chapter discusses the need to understand the pathways from social structure to health and the various attempts to identify these pathways, such as those outlined in the Commission on Social Determinants of Health (CSDH) report.