This book explains the design and development of international organization in the postwar period. It theorizes that the basic set up of an IO responds to two forces: the functional impetus to tackle problems that spill beyond national borders and a desire for self-rule that can dampen cooperation where transnational community is thin. The book reveals both the causal power of functionalist pressures and the extent to which nationalism constrains the willingness of member states to engage in incomplete contracting. The implications of postfunctionalist theory for an IO’s membership, policy portfolio, contractual specificity, and authoritative competences are tested using annual data for seventy-six IOs for 1950–2010.