The importance for foster children's well-being of good relations between foster parents and birth parents is a common topic of research. This article aims to contribute to an understanding of how co-parenting by foster parents and birth parents works in everyday life, from both parties' perspectives, whether or not they knew each other previously. The 10 studied cases, comprising altogether 19 interviews, concern teenage placements and are almost equally divided between kinship, network, and traditional foster families. This article claims that for co-parenting to be possible it is of vital importance to have an "open foster family", one that is open and welcoming toward the birth parents. Such openness includes the provision of regular information to the birth parents about the everyday life of their child, mutual planning of the child's situation, and, most beneficially, invitations for face-to-face encounters between youth, foster parents, and birth parents. Both parties' mutual engagement with the foster youth serves as the foundation of the cooperation. The service and support that social workers can offer in this process is important. Due to similarities between the family cultures, cooperation is facilitated in kinship foster families.