In this chapter, case studies of AIIB and Asian Development Bank (ADB) investments in countries of the ASEAN region are presented to demonstrate how the two institutions operate on the ground and how their interactions differ across sectors, project types, and national development contexts. It examines the role of host-country infrastructure priorities, debt and fiscal limitations, procurement and safeguard requirements, and domestic political factors in project design and financing decisions. The chapter also looks at the circumstances in which AIIB and ADB investments are complementary, i.e., by co-financing, sequencing, and division of labor, and when parallel activity, visibility competition, or strategic differentiation happens. The chapter offers tangible evidence to assess the presence of substitution, complementarity, or a more contingent relationship between institutional practices and the country’s constraints by following project-level processes and outcomes.