A long firm is a trading company set up for fraudulent purposes; the basic operation is to run the company as an apparently legitimate business, gradually extending the amount of cash advances from customers at the same time as increasing the amount of credit from suppliers; when the pot is large enough, the perpetrators decamp with customers' money and suppliers' goods. The procedure needs a certain amount of money to set up, often the proceeds from another crime or a previous long firm.

The Long Firm is a novel by Jake Arnott which describes a criminal's career; the title comes from the protagonist's first venture being a long firm.