Although many immigrant children to the United States arrive with their parents, a notable proportion are first separated and later reunited with their parents. How do the experiences of separation and reunification shape the well-being of immigrant children? Data were from a national survey of legal adult immigrants and their families, the New Immigrant Survey from 2003 to 2004 (for academic achievement, age 6-12, N = 876; for psychosocial well-being, age 6-17, N = 1,084). Results indicated that immigrant children who were once separated from their parents exhibited poorer literacy and higher risk of emotional and behavioral problems than those who migrated with parents. A protracted period of separation and previous undocumented status of parents amplified the disadvantages experienced by these children.