There are several provisions in the U.S. legislature that make monopolies illegal. For example:

The Sherman Anti-Trust Act states:

Every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States or with foreign nations, is declared to be illegal. Every person who shall monopolize, or attempt to monopolize, or combine or conspire with any person or persons to monopolize any part of the trade or commerce among the several State, or with foreign nations, shall be deemed guilty of a misdemeanor, and on conviction therefor, shall be punished by fine not exceeding fifty thousand dollars, or by imprisonment not exceeding one year, or by both said punishments, in the discretion of the court (McConnell 622).

The Clayton Antitrust Act elaborates on this by "Éoutlawing price descrimination between purchacers when such discrimination is not justified on the basis of cost differences" (McConnell 622). It also "Éforbids exclusive, or tying, contracts whereby a producer would sell a product only on [the] condition that buyer acquire other products from the same seller and not from competatorsÓ (McConnell 622). Further, it "Éprohibits acquisition of stocks of competing corporations when the effect is to lessen competition" (McConnell 622). Also, it "Éprohibits formation of interlocking directorates-the situation where a director of one firm is also a board member of a competing firm-in large corporations where the effect would beto reduce competition" (McConnell 622 & 623).

The Federal Trade Commission Act created the Federal Trade Commission, which was "charged É with the responcibility of enforcing the antitrust laws" and "was given the power to investigate unfair competative practices on its own initiative or at the request of injured firms. The Commission could hold public hearings on such complaints and, if necessary, issue cease-and-desist orders where unfair methods of competition in commerce were discovered" (McConnell 623).

Finally, the Celler-Kefauver Act prohibited one firm from obtaining physical assets of another firm, like plants or equipment, when the effect would be to lessen competition. (McConnell 623)

However, the government supports several existing monopolies, including the U.S. postal service (Kaz). So are monopolies illegal or not? It seems to me, that an illegal monopoly, is one that is not supported by the American government. Microsoft clearly fits this description, as it is currently being charged with an antitrust suit. The charges of this suit are: that it illegally tied it's browser, Internet Explorer, to Windows 98, that it used Windows' dominance to force retailers to sign exclusionary contracts, preventing them from promoting Netscape's browser, Navigator, and that it tried to collude with it's competitor, Netscape.