The government gives one person or firm the exclusive right to sell some good or service.

A company has control of a key natural resource.

The monopoly is established by the government.

Economies of scale are so large that one firm can supply the entire market at a lower average cost than two or more firms.

The usefulness of a product increases with the number of consumers who use it, making it difficult for other firms to enter into these markets and compete with existing firms.