Why do some states resist entering into international treaty regimes while others demonstrate eagerness to participate? Although factors such as degree of pressure exerted by international actors, ambiguity in the treaty language and a regime's 'lack of teeth' (enforcement and sanctioning mechanism) do affect participation, this book investigates whether internal (domestic) factors may ultimately be responsible for influencing why a state resists or joins international treaty regimes. The volume draws on United Nations treaty ratification data from three different issue areas - arms control, environment and human rights - to study the participation patterns of democracies and non-democracies in international treaty regimes. Incorporating two in-depth case studies on the United States and China, the author traces the impact of domestic institutional structure, state capacity and internal social norms on state decisions to resist or participate.