Since the late 1990s, moves towards regional integration and cooperation have gained momentum in East Asia. The regional countries have expanded and deepened integration initiatives under the ASEAN Plus Three (APT) framework that consists of ASEAN countries, China, Japan and South Korea. What factors have promoted the development of regional integration and economic cooperation in the region? This article addresses this question in terms of collectively shared norms and political leadership. Informality, a representative common norm, played a catalytic role in first nurturing communication for regional cooperation and inducing a reluctant state to join the cooperative framework. Importantly, the development of regional cooperation under the APT framework was accompanied by a shift in emphasis from informal to formal settings. Moreover, leadership shown by China and Japan has played a crucial role in promoting the regional integration initiatives. While China has taken the initiative in propelling regional free trade agreements and economic development and integration in the Indochina countries, Japan has taken the lead in developing financial and monetary architectures and other cooperative mechanisms. Rivalry for political leadership has induced the two countries to provide regional public goods in a positive-sum game manner.