The evidence examining the relationship between specific social factors and early childhood health and developmental outcomes has never been systematically collated or synthesized. This review aims to identify the key social factors operating at the household, neighborhood, and country levels that drive inequalities in child health and development. Medline and CHICOS (a European child-cohort inventory) were systematically searched to identify all European studies published within the past 10 y. 13,270 Medline articles and 77 European child cohorts were searched, identifying 201 studies from 32 European countries. Neighborhood deprivation, lower parental income/wealth, educational attainment, and occupational social class, higher parental job strain, parental unemployment, lack of housing tenure, and household material deprivation were identified as the key social factors associated with a wide range of adverse child health and developmental outcomes. Similar association trends were observed across most European countries, with only minor country-level differences. Multiple adverse social factors operating at both the household and neighborhood levels are independently associated with a range of adverse health and developmental outcomes throughout early childhood. The social gradient in health and developmental outcomes observed throughout the remaining life course may be partly explained by gradients initiated in early childhood.