As a result of the growth in out-of-wedlock childbearing and union instability, adults contemplating forming a new union are often already parents. This article examines the role of children in stepfamily formation, both coresident and not, using the 2,594 respondents in the National Survey of Families and Households who were not living with a partner in 1987/1988. We consider children of the respondent and of the partner, which allows us to examine the determinants of entering a stepfamily. We found that being a coresident father dramatically increases forming a union with a woman with children. Women’s coresidential children reduce women’s odds of forming unions with men who do not have children and increase them for unions with men who do.