This chapter demonstrates that the principle of the child’s best interests is an integral value of EU law and serves as an underlying rationale for EU legislation and judgments. It is analysed how the principle of the best interests of the child affects the rights of third country nationals in the EU, with a focus on decisions relating to family reunification and EU migration and asylum law. In this chapter a multidimensional conception of the child’s best interests principle is developed, emphasising its threefold function as a substantive right, as an interpretative tool and as a procedural rule. The article concludes that the principle of the child’s best interests will continue to grow in importance in EU law on third-country nationals.