Abstract: Originally construed to leave states room for manoeuvre in the security sector, the concept of margin of appreciation (MoA) has been developed in the jurisprudence of the European Court of Human Rights (ECtHR) to allow states leeway for action in all kinds of fields. It leads to a limitation of the Court’s supervisory powers where domestic authorities are better placed to take a decision and also justifies its self-restraint in areas where no international human rights standards exist (yet). Generally, the MoA can be considered a useful tool to unify 47 member states with different legal cultures under the supervision of the Court and leave them room for measures in accordance with local needs. However, as will be shown in view of the judgment S.A.S vs France, the MoA doctrine needs careful, principled application that is backed by sustainable reasoning. The inherent dangers of over-reliance might undermine the ECtHR’s position as final guarantor of the rights enshrined in the European Convention on Human Rights (ECHR) and strip applicants of effective protection of their human rights by the Court.