The literature on management of common-pool resources and social-ecological systems is extensive. It covers at least 40 years of research. In that time, significant progress has been made in understanding the conditions and factors that characterise sustainable management schemes, particularly community-based schemes. This thesis attempts to build on this literature. Its core concern and unifying theme is the ‘factors’ that explain why some management systems are successful and others are not. Of course, the list of factors identified in existing literature is extensive. Some of these factors - socio-economic heterogeneity, for instance - are explored in significant depth. This thesis does not propose to contribute to our understanding of such factors. Instead, it focuses on one factor – legitimacy – which is recognised as important by a large number of researchers, but has been subject to only limited investigation in the field. The thesis adopts a multi-disciplinary and mixed methods approach to ask how resource management systems - and the organisations that design and operate them - gain legitimacy. Much of the research is based on two case studies of resource management systems in Argentina and the United States. One of these systems is in the process of being implemented, but has been the source of much dissatisfaction to date. The other system, a fisheries management plan, was introduced in 1995, and has changed the fortunes of fishermen and other stakeholders in mostly positive ways. These case studies are the point of departure for developing a more complete picture of the factors that underpin legitimacy in resource management contexts.