Shelter theory is a general theory of small state behaviour in world politics. The vulnerability of small states following from their absolute and relative power creates an incentive for seeking shelter in an anarchic international system, where each state is left to cater for its own interests. Shelter theory offers a more fine-grained theory of state behaviour than balance of power theory. International anarchy is a self-help system creating incentives for small states to seek shelter in order to meet the challenges of insecurity, limited opportunities for growth and a small market place for ideas following from limited capacity and power asymmetry. Shelter theory may be viewed as an ideal type small state theory. It draws on what is at least implicitly a realist understanding of the special challenges following from smallness in international anarchy, but the theory nuances this view with liberal institutionalist assumptions about the effect of international institutions and a constructivist-inspired awareness of norms.