This chapter presents the Asian Infrastructure Investment Bank (AIIB) as a new multilateral development bank that opened in January 2016 under the leadership of China, amidst fissures in global infrastructure financing and controversies over whether it is a challenge or supplement to the established development finance system headed by such institutions as the Asian Development Bank (ADB) and the World Bank. It considers two prevailing interpretations, realist ones that focus on strategic rivalry and liberal ones that focus on functional complementarity, and contends that both of these approaches underestimate how interactions can produce convergence and differentiation within the same region. This chapter advances the book’s main argument: the relationship between the AIIB and ADB within ASEAN is best understood as conditional coexistence, shaped over time as a normal, practical adaptation among multilateral development banks considered social actors. These major-power interests in ASEAN are offered as a strategic testing ground where major-power interests collide, and where AIIB growth has become the subject of questions about competition, governance restructuring, and development outcomes. The chapter is a preview of the mixed approach of the book, systematic project analysis through a quantitative indicator and qualitative case analysis, to determine the nature of investment patterns as substitutes or complements and the way in which institutional relations change in practice.