Child support is produced by a formula, and almost everything that matters happens in the inputs rather than the arithmetic. This article explains the two guideline models, how income is defined far more broadly than a paycheck, and how the parenting time credit can move a number by hundreds of dollars a month. It then covers the two mechanisms that generate the most disputes: imputed income, where a court assigns earning capacity a parent does not actually have, and the deviation, where the guideline result is set aside for stated reasons. Modification receives extended treatment because the most expensive error in the field is waiting to file, given that support generally cannot be modified retroactively before the filing date. Enforcement sections cover income withholding, arrears and the interest that compounds them, contempt with its ability-to-pay requirement, license suspension, tax intercepts, and the federal machinery that makes interstate enforcement work. Worked numbers run throughout and every authority is linked.