This article develops the argument that informal institutions-'G-x groupings', 'contact groups', 'core groups'-provide a vital space to renegotiate the terms and conditions of US hegemony. With the global balance of power shifting, US hegemony today is no longer seen as the exclusive framework to solve urgent collective action problems-major armed conflict, nuclear non-proliferation, climate change, global financial stability. These problems are of global significance and litmus tests for two key properties of US hegemonic power: the ability to maintain order and to provide public goods. As the global financial crisis of 2008 has demonstrated, emerging powers consider US hegemony as part of the problem rather than the framework through which to develop solutions. The substantial challenges for liberal institutions to adapt to major shifts in the global distribution of power and to act as effective problem-solvers has led to growing recourse to alternative mechanisms of collective action that operate in and around the liberal institutional architecture. However, they are not the convenient support structure for the renegotiation of Western liberal order on exclusive US terms. Informal institutions are platforms for contestation with an open outcome. Contestation includes the possibility of developing new patterns and understandings that may ultimately replace Western liberal order. The social contract between hegemonic leader and followers can be revoked. The contract will be up for termination if there is a widely shared belief that the hegemon fails to deliver sufficient public goods and proves to be incapable of performing its system maintenance functions.