International organizations (IOs) in Asia and the Pacifi c have often been characterized as ‘informal’ in comparison to more ‘formal’ IOs. The latter organization is understood as the ‘proper’ type of IO defi ned by the existence of a permanent secretariat and a binding set of rules and/or agreements between governments. The European Union (EU) represents such a ‘formal’ IO that is regionally based. In this context, the Association of Southeast Asian Nations (ASEAN) is often contrasted with the EU as an ‘informal’ type, although scholars suggest it has become more formal in the past two decades. This chapter studies a long time span and examines the characteristics of IOs of the region, now called Asia and the Pacifi c.