This article introduces, summarizes and contextualizes the key questions and findings of a special issue of Global Policy on the resourcing of international organizations (IOs). The article sets out trends in the financial resources available to IOs; discusses their organizational consequences; and highlights analytical implications for the study of IOs. We discuss resource diversification associated with growing complexity of the origins and types of funding available to IOs; the importance of non-state actors in IO funding; and contestation over the classification of resources. Resource diversification encourages organizational differentiation, manifested in major shifts in resource-related actor constellations and their impact on the autonomy of IOs; adjustments to budgeting procedures; and functional differentiation within IOs and the emergence of new types of IOs that are partly driven by resourcing. These observations invite an analytical perspective in the study of IOs that pays systematic attention to the administrative governance dimension of IOs; the entrepreneurial character of many IOs; and organizational fields as a focus of analysis. Read together, the 11 contributions to the special issue underline that paying attention to their resourcing can advance our understanding of IOs.At a time when newly elected US President Donald Trump announces severe budget cuts to the United Nations system, the relevance of resourcing for the functioning of international organizations (IOs) can be in no doubt. It has long been recognized that access to financial and other resources is critical to the evolution of IOs and to the realization of their global policy ambitions (Wright, 1957). Complaints from the leadership of IOs about inadequate resources also have a long tradition, and the mismatch between IO mandates, as set by member states, and available resources to carry out those mandates effectively is a recurrent concern (see Annan, 1993, for the UN). Some IOs, such as the African Union (Engel, 2015) or UNRWA (Bocco, 2010) appear to suffer from chronic underfunding. Others have a long history of repeated funding 'crises', typically brought about by either the unexpected partial withholding of member state contributions (for the United Nations see Claude, 1964;Taylor, 1991; for UNESCO see H€ ufner, 2017;. In other cases, resource mismatch has been the result of unforeseen demands on IO budgets, as in the case of refugee crises or natural or man-made disasters (McDermott, 2000). In view of these challenges, the need to reform UN financing and resource politics has been recognized and many proposals have been put forward over the years (see Muttukumaru, 2016 for a present-day reform proposal).A central finding of this special issue is that there are several critical issues that matter when studying the resourcing of IOs, beyond the overall levels of funding available and the details of resource allocation. Among these issues are:(1) the mix of sources of funding, namely, who provides the finances of IOs; (2...