Globally, though to different extents, naturalisation remains of significant relevance to immigrants as this allows them to access rights restricted to nationals. However, not every state adopts the same rules to condition naturalisation and states with similar eligibility conditions sometimes have very different implementation practices. Focusing on residence-based naturalisation, this presentation takes policy differentiation seriously and aims to build up a systematic, comparative explanation for policy-variation in a global perspective. To assess the extent to which naturalisation policies vary an index is developed that allows determining how inclusive or exclusive individual countries are. This index looks both at the legal dimensions of naturalisation (eligibility requirements, procedural guarantees for applicants, protections against denaturalisation, etc.) and at its implementation dimensions (documentation to provide, use of discretion, levels of bureaucracy involved, etc.). The policy index should serve as an empirical starting point to test different theories explaining policy-variation in naturalisation in a more comprehensive manner than was the case in previous investigations, including theories that claim the existence of a ‘political regime effect’ in naturalisation policies or an impact of colonial legacies on the degree of inclusion of these policies.