The term health disparities (also called health inequalities) refers to the differences in health outcomes and related events across individuals and social groups. Social determinants of health, meanwhile, refers to certain types of causes of ill health in individuals, including lack of early infant care and stimulation, lack of safe and secure employment, poor housing conditions, discrimination, lack of self-respect, poor personal relationships, low community cohesion, and income inequality. These social determinants stand in contrast to others, such as individual biology, behaviors, and proximate exposures to harmful agents. This chapter presents some of the revolutionary findings of social epidemiology and the science of social determinants of health, and shows how health disparities and social determinants raise profound questions in public health ethics and social/global justice philosophy.