Using cohort data from the National Longitudinal Survey of Youth, this paper tracks the experiences of "serial cohabitors." Results indicate that only a minority of cohabiting women (about 15%-20%) were involved in multiple cohabitations. Serial cohabitations were overrepresented among economically disadvantaged groups, especially those with low income and education. They also were less likely than single-instance cohabiting unions to end in marriage rather than dissolve. If serial cohabitors married, divorce rates were very high--more than twice as high as for women who cohabited only with their eventual husbands. The results suggest the need to balance the government's current preoccupation with marriage promotion with greater support of "at risk" unions that marriage promotion initiatives have helped create.