International research data networks are critical for progress in many scientific domains and underpin efforts to promote Open Science. At the same time, many of these networks are fragile and the responsibilities for their support and performance are frequently distributed across a variety of different actors. This report explores the challenges and enablers for the effective functioning of international research data networks. It analyses the diversity and complexity of these networks, and issues such as governance and funding, in a selection of 32 cases. It includes a set of policy recommendations as a basis for building the shared understanding that is necessary to develop effective and sustainable international research data networks.