Research on social class inequalities in health has progressed from the work of late nineteenth-century social theorists into a strong stream within medical sociology and social epidemiology. Evidence has accumulated from various countries, and can be encapsulated into the health inequalities invariance, which stipulates that the lower the social class the poorer the health. Health inequalities are found across various sociodemographic subgroups, domains of social class (education, occupation, income, and wealth), and domains of health (self-reported health, functioning, somatic and mental diseases, and causes of death). Over time, health inequalities have either remained or widened. The causes of health inequalities include social-structural backgrounds, selection and social mobility, early and adult living conditions, lifestyles and health behaviors, psychosocial factors, and healthcare. Social class inequalities in health are largely avoidable and unfair. Policy programs suggest measures to tackle health inequalities.