This article examines how social workers approach and apply the principle of the child’s best interests in their practice when they make assessments in child welfare in Finland. The study rests on two types of data in which social workers describe their assessment practice: a survey of social workers with 373 responses and a vignette-based group discussion data with 120 practitioners in 28 groups. The findings highlight that the principle of the child’s best interests is acknowledged as being important and uncontested. However, the overall approach is procedural, focusing mainly on talking with children about their views and opinions. There is a weakness, if not even a lack of a substantive approach to the principle, with limited use of theoretical knowledge. The study suggests two ways forward: more reflective practices to critically explore the principle in its all variations in front-line practice, and a more theory-based approach to applying the principle.