All agreements and other collusive actions between undertakings which restrict competition and affect trade between Member States are prohibited according to the Article 101 of the Treaty on the Functioning of the European Union. It is necessary to fulfil certain conditions in order to apply the prohibition. One of them presumes the existence of the “agreement” formed by undertakings. The notion of the “agreement” covers agreements, decisions taken by associations and any other form of concerted practices developed by undertakings. The Treaty does not contain a definition of “agreement”, and therefore, the meaning of it has been defined within court practice. Due to the fact that it is extremely difficult to distinguish between situations in which undertakings in intelligent manner respond to reactions of their competitors and situations in which they advisedly act with knowledge about other’s behaviour, it is of the most importance to determine some common characteristics which should be taken into consideration when interpreting the notion of “agreement”. Having this in mind, the author in the article discusses and analyses the most important decisions of the Commission and judgments of the European Union courts with the effort to detect their common parameters. A special attention has been given to the category of concerted practices. Under this category all sorts of uncompetitive practices are included, for example the one which could not be embraced by the definition of the agreement or decisions taken by associations. It is especially interesting to see what has to be concluded with regard to unilateral actions. The author concludes that the concept of the “agreement”, as interpreted in court practice, means existence of will between at least two parties no matter the form of its manifestation if there is a clear expression of intent of the parties.