Following national transposition of procedural rights’ Directives, various eu and domestic authorities would need to interpret their provisions, including those concerning the right to custodial legal assistance. To inform this interpretation, account must be taken of the relevant ECtHR case law. However, many of the Directives’ provisions are so unclear, and the ECtHR approach is so inconsistent, that to achieve a coherent interpretation, it appears necessary to first identify the principles behind this right. This article examines these principles, in the form of rationales, from the theoretical, jurisprudential, and eu law perspectives. It concludes that the right should be conceived as grounded in a range of rationales, notwithstanding an alternative view that it serves (only) to protect the privilege against self-incrimination. The article fleshes out the consequences of this conclusion for interpreting some of the relevant Directives’ provisions, and for criminal defence practice.