This chapter concerns the normative dimension of legitimacy (including legal, participatory and expert). This is because the descriptive dimension of legitimacy requires empirical analysis, and it is highly challenging to make an objective measurement in such analysis for legitimacy issues. So, instead of conducting research on outcomes of procedures through examination of submissions on non-compliance cases and decisions on them, or the perceptions or beliefs of parties on the justification of authority, it inquires the justification of authority provided in accordance with law, or transparent and participatory processes or based on experts’ decisions under such procedures in proper to the concept of normative legitimacy. It focuses on the conceptual and theoretical basis on both compliance mechanisms(CMs) under multilateral environmental agreements(MEAs) and legitimacy. Then it examines such procedures under these mechanisms in terms of legitimacy concerns in five areas : legal basis of decision-making power; legal basis of non-compliance procedure (NCP); the institution of the NCP; procedural safeguards; and the legal character of measures applied to non-compliant parties.