Arrears (also sometimes known as rearage) 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 arrears is a term describing payments made after a service has been provided. In this case use of the word "arrears" does not imply any breach.

The term in arrears is also used in many contexts to refer to payments made at the end of a period, 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 arrears (the interest for the period is due at the end of the period). Employees' salaries are usually paid in arrears.

In other contexts, 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 90 days is said to be three months in arrears. Precise usage may differ slightly (e.g. "in arrear" or "in arrears" for the same situation) in different English-speaking countries.