Soon after the boards of Ticketmaster and Live Nation approved a merger, the U.S. Department of Justice opened an investigation into what that would mean. No wonder: The combined company, to be called Live Nation Entertainment, would be a ticketing, promotion and merchandise superpower with interests in every area of the music business.

But even superpowers are subject to regulatory review. Under antitrust laws, the DOJ and the Federal Trade Commission (FTC) can review and challenge mergers that would create an unfair competitive advantage that could hurt consumers. Usually, when the companies that plan to merge are a certain size, they notify those agencies, which start a 30-day screening. After that, the merger can be subjected to a "second request for more information," a protracted data collection process that can involve exhaustive document reviews and considerable legal wrangling.

Essentially, the government looks for indications that the proposed merger will create an unfair competitive advantage or prove harmful to the public interest. Then it either opposes the deal or exacts concessions.

Proposed deals are usually analyzed as "horizontal" mergers (between competitors) or "vertical" ones (between buyers and sellers). The FTC believes that most mergers benefit consumers by allowing firms to operate more efficiently. But mergers that lessen competition can lead to higher prices, reduced availability of goods and services, lower quality of products and less innovation -- and are less likely to withstand regulatory scrutiny.

Click here to read more about the Ticketmaster and Live Nation merger, including how it could become a test balloon for the Obama administration's antitrust policy.