Carnegie and Rockefeller were very instrumental in the development of business in the United States. Carnegie sought to control all of the aspects of his business, from the iron ore to the coal needed to fire his furnaces. Carnegie even created company towns such as Homestead for his workers. Rockefeller undercut his competitors' prices so that he could then buy out his competitors. He also used the railroads by refusing to do business with any rail line that carried a competitor's oil.

Both of these men would create monopolies that would drive out the competition. This was important in American business because at one time during the Gilded Age, one could argue that these two men's corporations had more operating capital than the United States government. While their businesses made the United States a major player in the steel industry and the refined oil markets, they also wielded considerable power by not allowing any competition. It was because of Rockefeller and Carnegie that Congress passed the Sherman Antitrust Act. Even with the breakup of Standard Oil and the sale of US Steel to J. P. Morgan, the U.S. continues to be the home of major businesses today that dominate industries such as technology.

Entrepreneurs like Andrew Carnegie and John D. Rockefeller helped to make America's economy into one that was dominated by big businesses, some of which practically controlled entire industries.  Before they and others like them came on the scene, there was much less in the way of really big companies in the United States.  But men like these were very entrepreneurial and were constantly in search of new ways to make profit.  This led them to create ever larger companies in search of economies of scale.  Starting with these men, the US economy has been dominated by large businesses.