"Contested multilateralism" describes the situation that results from the pursuit of strategies by states, multilateral organizations, and non-state actors to use multilateral institutions, existing or newly created, to challenge the rules, practices, or missions of existing multilateral institutions. It occurs when coalitions dissatisfied with existing institutions combine threats of exit, voice, and the creation of alternative institutions to pursue policies and practices different from those of existing institutions. Contested multilateralism takes two principal forms: regime shifting and competitive regime creation. It can be observed across issue areas. It shapes patterns of international cooperation and discord on key security concerns such as combating terrorist financing, halting the proliferation of weapons of mass destruction, and banning certain conventional weapons. It is also evident on economic issues involving intellectual property, on environmental and energy issues, and in the realm of global public health. The sources of dissatisfaction are primarily exogenous, and the institutions used to challenge the status quo range from traditional treaties or intergovernmental organizations to informal networks, some of which include non-state actors. Some institutions are winners from the process of contested multilateralism; others may lose authority or status. Although we do not propose an explanatory theory of contested multilateralism, we do suggest that this concept provides a useful framework for understanding changes in regime complexes and the strategies that generate such changes.