The rational choice methodological approach is informed by economics and assumes that actors seek to achieve goals amidst certain constraints. Rational choice helps us think about why states might choose to work through the G20, which has distinct design characteristics amidst a broader spectrum of global governance organizations. This chapter argues that the G20 is an informal intergovernmental organization (IIGO) because it does not have a legalized treaty or a permanent independent secretariat. It is rational for states to choose the G20’s informality because it provides flexibility, reinforces state autonomy, helps control information, speeds up decision-making processes, and minimizes bureaucratic costs. These benefits are particularly advantageous in managing high uncertainty or crisis scenarios that have become central for world leaders today. Nonetheless, rational choice also helps us understand that the G20’s informality is not a panacea and discusses ways that the G20 might be challenged in the years to come.