The institutionalization of mountain regions in Europe, Regional Studies. Since the 1990s, many ‘project regions' have emerged in Europe, a trend deriving from the tendency to adopt ad hoc institutions for specific spatial and environmental issues and the empowerment of diverse stakeholders who compete with the dominant role of states. This article addresses the building of institutionalized mountain regions in Europe, analysing how the specificity of mountain areas was considered in policies by states and the European Union, and later for transnational mountain ranges. Environmental and trans- boundary issues have been major driving forces for new institutional arrangements combining a territorial and environmental focus and complex networks of stakeholders.