The principle that ‘in all actions relating to children (…) the child's best interests must be a primary consideration’ (Article 24(2) EU Charter of Fundamental Rights) is widely applied by the Court of Justice of the European Union (CJEU). This article considers preliminary rulings in which the CJEU had recourse to the best interests of the child as the criterion for settling the dispute in the main proceeding. For analytical purposes, these cases may be grouped in two clusters: cases resembling child custody disputes, and other cases. It is argued that, in the light of the varied factual circumstances of the judicial disputes, the application of the criterion of best interests of the child as primary consideration warrants further scrutiny. To this end, this article identifies and assesses a practical and a conceptual challenge to the widespread application of the criterion. The former posits that the criterion is indeterminate, and this article considers whether drawing from Article 3 of the Convention on the Right of the Child may help in solving the uncertainty; the latter maintains that the test is unjust and self-defying, a critique that this article finds only partially founded, for cases resembling child custody disputes.