Internal migration has resulted in a large number of left-behind children in China. Despite growing attention to this population, important gaps remain in our understanding of their cognitive development and the factors that mediate the impact of migration on children. The present study draws on a new nationally representative survey of Chinese children to study the psychological and cognitive development of left-behind children. Results show that rural children left behind by both parents (but not by one parent) are worse off in both psychological well-being and cognitive development than rural children living with both parents. The disadvantage of left-behind children is mediated by their caregivers' emotional well-being, parenting practices, and education. We also find a pronounced rural-urban difference in children's cognitive development.