Monopolies in the United States have existed in many forms. When a business dominates a market, its market power makes it a monopoly. How these businesses use their market power will determine the legality of the monopoly. Contrary to popular belief, monopolies are not illegal in the United States. What is illegal is actions taken by monopolies to limit competition. This lesson will use the railroad industry and the computer industry to show how monopolies gain their power. Lesson von EconEd (2006)