A promissory note is a written promise to pay a debt. An unconditional promise to pay on demand or at a fixed or determined future time a particular sum of money to or to the order of a specified person A promissory note should have several essential elements, including the amount of the loan, the date by which it is to be paid back, the interest rate, and a record of any collateral that is being used to secure the loan. Default terms (what happens if a payment is missed or the loan is not paid off by its due date) should also be spelled out in the promissory note.