Monopolies, a market structure with a single seller and no close substitutes, wield significant market power. They form through legal barriers, resource control, or economies of scale, allowing them to set prices above marginal cost and earn long-term profits. Unlike perfect competition, monopolies face downward-sloping demand curves and create inefficiencies. Governments regulate monopolies through antitrust laws and price controls to promote competition and protect consumers. Real-world examples include utilities, sports leagues, and tech giants.