Overdue refers to materials that have not been returned to a library by the date they were due. When an item becomes overdue, the borrower may be charged a fine or fee. The amount of the fine or fee can vary depending on the type of material and the policies of the particular library. Some libraries have automated systems that send reminders to patrons when an item is overdue or about to become overdue. Overdue materials can also cause problems for other patrons who may be waiting for the item, so it is important for borrowers to return materials on time.