In litigation proceedings before regional human rights Courts and Commissions, only applications meeting admissibility requirements can reach the merits stage. While claims’ admissibility is an established notion in public international law, human rights regimes have developed rules and implementation approaches that are specific to this field of litigation. A comparative analysis of the case-law of the four regional human rights systems reveals that the law and practice of admissibility has a common function across the different models, which transcends the idiosyncrasies and historical trajectory of each. In establishing and applying admissibility requirements, the regional human rights systems try to strike a balance between the individual’s access to justice and the viability of their work.