The concept of the “essence”—as well as the related concepts of “substance” or “core”—of fundamental rights is absent from the text of the European Convention on Human Rights (ECHR), but regularly appears in the case law of the European Court of Human Rights (ECtHR) since the Belgian Linguistic case of 1968. Yet, fifty years after its explicit emergence in the Convention’s legal order, it must be observed that a clear understanding of this concept and of its practical utility is still lacking. Indeed, the idea of the essence of fundamental rights has never been clearly defined in its case law, which remains essentially pragmatic and unprincipled in this field. This Article will therefore attempt to remedy this shortcoming by sketching out the different functions assigned to the concepts of the essence, substance, and core of rights in the ECtHR’s case law. It is postulated that the concepts of the essence, substance, and core of fundamental rights are invoked for three different types of purposes. First, the concepts of the essence, substance, and core are—apparently at least—used by the ECtHR to fix the “limit on the limits,” for example, the inalienable part of fundamental rights safeguarded from any possible restriction. Second, this concept has been a vehicle for expanding the Convention’s sphere of protection for the purposes of guaranteeing its effectiveness. Third, the concepts of the essence, substance, and core of fundamental rights also constitute a “reviewing tool” used by the Court to determine the intensity of the States’ obligations on the basis of a prioritization among a series of values at stake. Although these three different functions can be identified on paper, the practical usefulness, workability, and desirability of the concepts of the essence, substance, and core will be questioned.