Arrears (or arrearage) is a legal term for the part of a debt that is overdue after missing one or more required payments. The amount of the arrears is the amount accrued from the date on which the first missed payment was due. The term is usually used in relation with periodically recurring payments such as rent, bills, royalties (or other contractual payments), and child support.

Payment in arrear is a payment made after a service has been provided, as distinct from in advance, which are payments made at the start of a period. For instance, rent is usually paid in advance, but mortgages in arrear (the interest for the period is due at the end of the period). Employees' salaries are usually paid in arrear. Payment at the end of a period is referred to by the singular arrear, to distinguish from past due payments. For example, a housing tenant who is obliged to pay rent at the end of each month, is said to pay rent in arrear, while a tenant who has not paid rental due for 30 days is said to be one month in arrears. Precise usage may differ slightly (e.g. "in arrear" or "in arrears" for the same situation) in different countries.