Since the European Court of Human Rights (Court/ECtHR) began to define an autonomous concept of penalty, its case law has been developing on the basis of the Engel criteria. This study aims to reveal the implementation of these criteria by the ECtHR and its consequences under the case law. Although the definition of penalty depends on the application of these criteria, the existence of a problem of consistency among them draws attention. As a matter of fact, one of the criteria, the nature of the sanction has undertaken a function to expand the Court’s ratione materiae through an objective assessment. However, the other Engel criteria, nature of the offence and degree of severity of the sanction are open to criticism in terms of objectivity, as well as narrowing the limits of the concept of penalty and consequently restricting the Court’s ratione materiae.