The Sherman Antitrust Act of 1890, more commonly referred to as the Sherman Act, is a key piece of U.S. federal antitrust legislation. Section 2 of the Sherman Act deals with monopoly conduct. The goal is to protect free trade and commerce by restricting unfair competition. Section 2 also forbids exclusionary conduct.

Section 2 of the Sherman Act focuses on unilateral (single-firm) conduct. This conduct involves actions taken by a company to attain or maintain monopoly power. Section 2 can also target combined (or coordinated) conduct. This conduct involves a conspiracy to attain or maintain monopoly power.

The plaintiffs in a Section 2 case can be U.S. federal antitrust agencies, state attorney generals, or private parties. Private parties must first establish standing to bring a Section 2 claim. Since Sherman Act violations also violate the Federal Trade Commission Act, the Federal Trade Commission (FTC) also has the authority to challenge monopolistic activity.

In order to bring a success monopolization claim, a plaintiff must establish the presence of a number of elements. The plaintiff must clearly define the relevant market, whether it is a product market or geographic market, over which the defendant allegedly exerts monopoly power. The plaintiff must also demonstrate that the defendant willfully attained monopoly power through exclusionary tactics. Finally, the plaintiff must show actual possession of monopoly power by the defendant.

Expert testimony may be used to define the relevant market. To define a product market, a demand-side analysis or a supply-side analysis can be used. A demand-side analysis looks at whether products are reasonably interchangeable in use in the event of a price change. The more substitutable products are, the more products are considered to compete with each other in the same product market. A supply-side analysis looks at all the potential suppliers of the product in question. It assesses the likelihood that such potential suppliers could begin to supply the product in a timely manner in the event of a price increase. Such theoretical suppliers are included in the relevant product market if they could begin supplying sufficient quantities of the product in a timely manner.

Courts have developed standards for assessing different types of exclusionary conduct. Examples of exclusionary conduct include vertical restraints to limit competitor access to customers, supplies, or markets, tying arrangements, predatory pricing, and product disparagement.

In practice, Section 2 cases are relatively uncommon. In 1998, the U.S. Department of Justice (DOJ) brought a lawsuit against Microsoft, alleging that Microsoft was illegally monopolizing the PC operating system market. While an initial ruling upheld this claim, a settlement was reached between the parties in which Microsoft agreed to reform some of its business practices.

In 2020, the DOJ brought a lawsuit against Google, alleging that the Google violated Section 2 of the Sherman Act by engaging in monopolistic behavior. The DOJ claims that Google used anticompetitive tactics in search advertising and to buy preferential treatment of its search engine. The U.S. District Court for the District of Columbia ruled that Google violated Section 2 and prohibited Google from entering into or maintaining exclusive contracts relating to search.

While remedies in Section 2 cases can consist of civil penalties or injunctions, injunctive relief is more common. Such injunctive relief usually is in the form of conduct remedies, rather than structural remedies such as the forced divestiture of a business line.