Best interests (of the child) describes the overarching welfare test used by courts, tribunals and public authorities when making decisions affecting children, including disputes about residence and contact, care proceedings, adoption, medical treatment and relocation. It requires decision‑makers to treat the child’s welfare as the central, and often paramount, consideration, and to assess what outcome will best promote the child’s overall wellbeing, safety, development and rights in the particular circumstances. In England and Wales, the concept is framed by the “paramountcy principle” in the Children Act 1989, supported by a statutory welfare checklist and extensive case law. Northern Ireland adopts a similar approach under the Children (Northern Ireland) Order 1995. In Scotland, the Children (Scotland) Act 1995 and subsequent legislation require the child’s welfare to be the court’s paramount consideration, informed by views of the child where practicable. In Ireland, the best interests of the child are constitutionally protected and elaborated in the Children and Family Relationships Act 2015 and the Guardianship of Infants Act 1964, as amended. Across all jurisdictions, the assessment is holistic, child‑specific and fact‑sensitive, rather than a fixed formula or checklist alone.