The term "antitrust" refers to competition rules on agreements and business practices which restrict competition and on abuse of dominant positions.

Agreements and concerted practices which may restrict competition are prohibited by the antitrust provisions of Article 101 of the Treaty on the Functioning of the European Union - TFEU. The prohibition applies to cartels (also referred to as "agreements") and relates to situations in which competing businesses collude to restrict competition, by fixing prices, limiting production or sharing markets. Restrictive agreements may nonetheless be permitted if they generate more positive than negative effects (agreements improving production, product distribution, contributing to technical progress, etc.).

The Community antitrust rules also forbid abuse of a dominant position within a market (Article 102 of the TFEU). An undertaking is in a dominant position when it has a substantial proportion of a market and can evade normal competition on it. It is then forbidden to make abusive use of its dominance, e.g. by charging excessive or unduly low prices or practising discrimination between commercial partners.

The Commission may impose large fines on undertakings engaging in illegal business practices.

Since 1 May 2004, the updated Community antitrust rules have allowed the national competition authorities to enforce the Community rules on agreements and dominance abuse in the same way as the Commission.