On the occasion of the tenth anniversary of the judgment in Muršić v. Croatia, delivered by the Grand Chamber of the European Court of Human Rights on 20 October 2016, this study seeks to define the contours of the European notion of prison overcrowding in breach of the human rights of detainees. This notion is essentially case-law based, since neither Article 3 of the European Convention on Human Rights nor Article 4 of the Charter of Fundamental Rights of the European Union defines inhuman or degrading treatment. The article traces the origins of the Muršić judgment, analyses its criteria – the 3 m² and 4 m² thresholds, the method for calculating floor space, the strong presumption of a violation and the compensating factors – and examines how they have been received in the subsequent case-law of the ECtHR, including the most recent, and in that of the Court of Justice of the European Union. What emerges is a common European case-law definition, stable in its essential elements despite some adjustments and certain unresolved tensions, which appears to strike a serious and reasonable balance capable of ensuring effective compliance with Article 3 of the Convention.