This article reviews the European Court of Justice’s case-law on the so-called principle of reasonableness. Many authors represent reasonableness as the rule of reason transferred to free movement of goods, persons and services, and applied more generally to European citizenship and competition. The idea is here contended, as the Court never accepted the parties argument that reasonableness may be considered an autonomous parameter of legitimacy of national or Union acts or norms. In free movement of goods or services cases, the Court always used the proportionality test, not mentioning the reasonableness or the rule of reason invoked by the parties. The Cassis de Dijon formula is something different from the rule of reason. In competition cases, the use of the rule of reason has been expressly excluded by European judges, as the economic approach has been applied not only to Article 101 (3) but also to Article 101(1) already in early case-law. The only autonomous and specific use of reasonableness as a legitimacy parameter is related to the delay of process and administrative procedures, particularly when it is linked with the fundamental right of defence.