International networks of non-governmental organisations have assumed increasing importance in global politics over the past two decades. Attention to issues of environment and sustainable development in particular present a strong rationale for their engagement as active participants at every level of decision-making. Over the same period, significant advances in communication technology have changed the nature of global dialogue, and made it possible for organisations to interact globally in new ways. However, many International Relations theorists consider Non-Governmental Organisations (NGOs) as significant actors principally to the extent that they influence inter-governmental deliberations. This ignores a wealth of material which illustrates the diversity of objectives NGOs prioritise in the global polity. In particular, the functions and principles evident in the work of international NGO networks suggest much more complex and diverse goals and ways of working. This thesis presents a novel consideration of ways in which interaction between NGOs collaborating internationally is significant. It explores the different functions international NGO networks exist to perform, and the ways in which these challenge established understandings of the role of nongovernmental actors in global governance. Attention is also given to the distinctions between issue specific networks, established to enhance collaboration in particular policy areas, and broader networks which attempt to transcend these divisions. Problems and tensions which can arise within international NGO networks are also addresse& The thesis includes a detailed study of international NGO networking before, during and after the UN Conference on Environment and Development (UNCED) in 1992. It draws on an extensive array of primary material from the UNCED process and subsequently which has not previously been widely available, and considers ways in which 'sustainable development' has been interpreted by organisations which exist to challenge prevailing economic and social norms