Competition law is the field of law that promotes or seeks to maintain market competition by regulating anti-competitive conduct by companies. Competition law is implemented through public and private enforcement. It is also known as antitrust law (or just antitrust), anti-monopoly law, and trade practices law; the act of pushing for antitrust measures or attacking monopolistic companies (known as trusts) is commonly known as trust busting.

The history of competition law reaches back to the Roman Empire. The business practices of market traders, guilds and governments have always been subject to scrutiny, and sometimes severe sanctions. Since the 20th century, competition law has become global. The two largest and most influential systems of competition regulation are United States antitrust law and European Union competition law. National and regional competition authorities across the world have formed international support and enforcement networks.

Modern competition law has historically evolved on a national level to promote and maintain fair competition in markets principally within the territorial boundaries of nation-states. National competition law usually does not cover activity beyond territorial borders unless it has significant effects at nation-state level. Countries may allow for extraterritorial jurisdiction in competition cases based on so-called "effects doctrine". The protection of international competition is governed by international competition agreements. In 1945, during the negotiations preceding the adoption of the General Agreement on Tariffs and Trade (GATT) in 1947, limited international competition obligations were proposed within the Charter for an International Trade Organisation. These obligations were not included in GATT, but in 1994, with the conclusion of the Uruguay Round of GATT multilateral negotiations, the World Trade Organization (WTO) was created. The Agreement Establishing the WTO included a range of limited provisions on various cross-border competition issues on a sector specific basis. Competition law has failed to prevent monopolization of economic activity. "The global economy is dominated by a handful of powerful transnational corporations (TNCs). ... Only 737 top holders accumulate 80% of the control over the value of all ... network control is much more unequally distributed than wealth. In particular, the top ranked actors hold a control ten times bigger than what could be expected based on their wealth. ... Recent works have shown that when a financial network is very densely connected it is prone to systemic risk . Indeed, while in good times the network is seemingly robust, in bad times firms go into distress simultaneously. This knife-edge property was witnessed during the recent (2009) financial turmoil. ..."