The law in most developed countries puts ‘the best interests of the child' at the heart of decisions about children, and practitioners use this standard in their work. In this article, the views of lawyers and social workers from England and France on the best interests principle are presented: practitioners define best interests and apply it to three examples of private law disputes over children. While they are largely united in their views on the factors which make up children's best interests, practitioners vary, both cross-nationally and between the two professions involved, in their application of best interests to particular cases.