Family instability is often offered as an explanation for the possible disadvantages faced by children with cohabiting parents. However, measures of family stability typically have focused on changes in legal marital status to the exclusion of cohabitation status. Consequently, children’s experiences of family instability have been underrepresented (Raley and Wildsmith 2004). To better understand a key mechanism influencing the well-being of children in cohabiting parent families, 66 in-depth interviews of cohabiting parents and/or cohabiting stepparents from the Cohabitation and Marriage in America project are analyzed, with particular focus on changes in residential composition experienced by their children. We use these rich data to examine the nuances of children’s experiences of stability and fluidity among those who have been exposed to cohabiting parent families. Based on our analysis two themes emerged that may be important considerations in our understanding of family instability: definitions of family stability and shifting family and household membership. (Author abstract)