The suitability test, the ‘rational connection’ prong of the proportionality test, asks whether a rights-limiting measure can advance its aim. Drawing on counterterrorism measures, whose effectiveness is seldom scrutinized, this thesis models the test under the European Convention on Human Rights. It distinguishes three functions. First, rational connection must turn on expected value (probability of success and magnitude of benefit), not probability alone. Second, the justifiability of overinclusive measures (reaching beyond what they centrally target) is a matter of the marginal benefit produced by their wider scope. Third, underinclusive measures (drawn more narrowly than they might be) must be justified by substantive reasons aligned with their scope, not merely the absence of improper motives. The thesis then maps the European Court of Human Rights’ engagement with means-ends analysis across its jurisprudence and proposes suitability as a Convention standard binding on all actors in rights adjudication, the Court included.