Inspired by Merton and Barber’s sociological theory on ambivalence, this article analyses ‘co-parenting’ between foster parents and birth parents as prototypes of ambivalent relationships; that is, relationships based on incompatible role requirements. This incompatibility is rooted in the conflicts between (a) the professional role of foster carers and their emotional involvement in the child in their care, and (b) the status of birth parents as ‘failed parents’ (from the perspective of the authorities) and their continuous aspirations to get their child home again. The article is based on qualitative interviews with foster parents and birth parents of children in foster care in Denmark. We show how the structural ambivalence is associated with difficulties, for both foster parents and birth parents, in translating the principle of ‘the best interest of the child’ into concrete practice in out-of-home placements.