The discussion on the Asian Infrastructure Investment Bank (AIIB) vs. the Asian Development Bank (ADB) often ends in the same place: competition over power politics or collaboration to finance and cooperate. The chapter re-articulates that discussion by treating the AIIB-ADB relationship as a dynamic process that may change over time, across sectors, and in different operating environments. It reviews the two dominant plots that inform much of the literature, including power-based interpretations and cooperation-based interpretations, and demonstrates why each of them is incomplete when the empirical record presents both convergence and differentiation. Next, in the chapter, a pragmatic prism is formed to understand the interaction of multilateral development banks (MDBs): how mutual expectations, legitimacy, and an institutional identity are formed through recurrent interaction, and how this interaction can stabilize what is referred to in this book as conditional coexistence within a common regional governance space.