Custody is decided under a standard that contains no rule. The best interests of the child is a direction to a judge, not a test with an answer, and it produces outcomes that vary with the judge, the county, and the evidence available. This article explains what the standard actually consists of in practice: the statutory factor lists every state now supplies, the constitutional floor that Troxel v. Granville places under a fit parent's decisions, the presumptions and burdens that do the real work including domestic violence presumptions and the joint custody presumptions a majority of states have adopted, and the evidentiary machinery of guardians ad litem, custody evaluations, and the child's own preference. It then covers the two contexts where the standard is hardest — relocation, where any outcome imposes a substantial cost on someone, and interstate and international disputes governed by the UCCJEA and the Hague Abduction Convention. The second half works child support: the guideline models states use, how income is determined and imputed, the deviations courts will and will not accept, modification, and the enforcement apparatus that operates largely without a courtroom.