The aim of this essay was to study how social workers apply the child's best interests regarding the principle of reunification and to investigate their work process around interventions in connection with the reunification process. This essay is a qualitative study where we have done seven semi-structured interviews with social workers who work with placement of children in foster care. In the essay we have conclude that it is a complex job that the social workers do when they must relate to the child's best interests in reunions. It is about getting to know the specific child's situation to understand what is best for each individual child. The child's best interest is a concept that is difficult to interpret because it is partly so individual for each child and because there are different perceptions of what it means. Even if reunions are a principle, it is not always realistic. They need to work on individually adapting the work to the family's needs and conditions. The reunification process can be different depending on the social worker. There are several factors that affect the conditions for reunification and no guidelines on how to proceed in the event of reunification.