Previous research on cohabitation and child well-being has primarily used cross-sectional analyses to understand how children in cohabiting families fare vis-à-vis their counterparts in other family types. So, we know little about transitions in cohabiting families and how these transitions might affect children’s development over time. In this paper, we use the Kindergarten Cohort of the Early Childhood Longitudinal Study (ECLS-K) to examine how transitions out of cohabiting family arrangements may affect young children’s cognitive development. Our preliminary analyses suggest that transitions do not affect the development of young children who initially reside in cohabiting, two-biological parent families. In contrast, for children initially living in a cohabiting stepfamily, we observe a faster rate of cognitive development for children whose mother marries by the end of first grade. However, our multivariate analyses suggest that this “”marriage-benefit”” is a function of the family’s economic resources. (Author abstract)