The more competition there is, the more likely are firms to be efficient and prices to be low. Economists have identified several different sorts of competition. Perfect competition is the most competitive market imaginable in which everybody is a price taker. Firms earn only normal profits, the bare minimum profit necessary to keep them in business. If firms earn more than this (excess profits) other firms will enter the market and drive the price level down until there are only normal profits to be made. Most markets exhibit some form of imperfect or monopolistic competition. There are fewer firms than in a perfectly competitive market and each can to some degree create barriers to entry. Thus firms can earn some excess profits without a new entrant being able to compete to bring prices down. The least competitive market is a monopoly, dominated by a single firm that can earn substantial excess profits by controlling either the amount of output in the market or the price (but not both). In this sense it is a price setter. When there are few firms in a market (oligopoly) they have the opportunity to behave as a monopolist through some form of collusion (see cartel). A market dominated by a single firm does not necessarily have monopoly power if it is a contestable market. In such a market, a single firm can dominate only if it produces as efficiently as possible and does not earn excess profits. If it becomes inefficient or earns excess profits, another more efficient or less profitable firm will enter the market and dominate it instead.