A sum is “payable on demand” when the debtor must pay as soon as the creditor asks for payment, without any further time period needing to expire. It is commonly used in loan agreements, guarantees, promissory notes, negotiable instruments and certain banking and security documents across England and Wales, Scotland, Northern Ireland and Ireland, with broadly consistent meaning.In practice, a debt payable on demand is immediately due once properly demanded under the contract or instrument (for example, by written notice, or presentation of a demand note). No fixed repayment date is required; the creditor controls timing by choosing when to demand, subject to any express contractual notice requirements, good faith duties and statutory protections (such as consumer credit or unfair terms regimes).Case law in the UK and Ireland has considered what constitutes an effective demand and when time starts to run for limitation or prescription purposes in “on demand” debts, which can differ from contracts with a specified maturity date. Practitioners should draft demand mechanics clearly, including method and place of demand, and be alert to any regulatory or equitable constraints on enforcing “payable on demand” obligations.