The act, based on the constitutional power of Congress to regulate interstate commerce, declared illegal every contract, combination (in the form of trust or otherwise), or conspiracy in restraint of interstate and foreign trade.
The ShermanAct authorized the federal government to institute proceedings against trusts in order to dissolve them, but Supreme Court rulings prevented federal authorities from using the act for some years.
Antitrust legislation is primarily regulated by the Antitrust Division of the Dept. of Justice and the FTC.
The Act was not used in court cases for some years, but Theodore Roosevelt used the Act extensively in his antitrust campaign, managing to divide the Northern Securities Company.
Some alleged violations of the ShermanAct are not prosecuted criminally, but rather are adjudicated in civil proceedings under a "rule of reason" standard, which examines the economic benefits and harm of allegedly anticompetitive conduct to determine whether it is, on balance, beneficial to consumers and should be permitted to continue.