Perfect competition is the opposite of a monopoly, in which only a single firm supplies a particular good or service, and that firm can charge whatever price it wants because consumers have no alternatives and it is difficult for would-be competitors to enter the marketplace. Under perfect competition, there are many buyers and sellers, and prices reflect supply and demand. Also, consumers have many substitutes if the good or service they wish to buy becomes too expensive or its quality begins to fall short. New firms can easily enter the market, generating additional competition. Companies earn just enough profit to stay in business and no more, because if they were to earn excess profits, other companies would enter the market and drive profits back down to the bare minimum.