Since parental reunification is the goal for most Swedish children in foster care, it is normally important to maintain contact with parents. However, sometimes a foster child’s best interests may be to stay in the foster home through a transfer of custody. This study set out to identify the legal principles that emerge when courts interpret a foster child’s best interests. A theoretical framework of normative legalism and psychology was used. The study shows that attachment theory often is emphasized in the legal material, and that precedents means there are a great many legal principles of relevance when a fosterchild’s best interest is to be assessed. Some of these principles are flexible, whereas other principles might collide. The conclusion of the article is that the occurrence of the word attachment in assessments and legal texts may not always refer to the psychological attachment theory. The results also suggest that actions of the authorities are not scrutinized, and that courts often give preference to the authorities’ interpretation of situations.