Rapid declines in common pool resources worldwide have been a driver to shift fisheries management from government to co-management. During the last few decades, theoretical and practical issues of fisheries co-management have been widely documented, yet the building and implementation of fisheries co-management in developing country contexts, where co-management processes have been influenced by external agencies, has not been adequately addressed. This research aims to contribute to understanding about how co-management works in a developing country and to building successful fisheries co-management in Vietnam.In particular, it investigates how fisheries co-management emerged and became arranged; it analyses the factors affecting the development and implementation of fisheries co-management and the outcomes of the co-management. This research was conducted in Tam Giang Lagoon, Vietnam where six nested case studies, fishing communities -represented by their Fishery Associations -were selected for the research. Data were collected from a survey (n=252) and 12 focus group discussions with fishers; semi-structured interviews with 27 people who were resource managers, co-management experts, project staff, policy makers and fishers; and secondary data where available. The findings show that (1) Comanagement in Tam Giang Lagoon was arranged between fishers' organizations (Fishery Association -FA) and district government, with low involvement of the provincial level of government, which is responsible for fisheries policy, planning and legislation, in co-management processes. Co-management has evolved incrementally as a response to problems, rather than co-management being a consciously designed or negotiated system from the start. Further, the development of the co-management system was led by donor-funded projects and their involvement was crucial for starting co-management in the lagoon. However, the dominance of their project teams in building the co-management has left government in a passive position. The low participation of government in co-management processes, poor cooperation between the many different donor-funded projects, and lack of appropriate exit strategies of the projects, have resulted in an unsustainable and inconsistent co-management system. (2) Co-management has offered the opportunity to shift the fisheries governance into a new era where power can be ii shared between government and fishers. However as the national and provincial legislative framework has lagged behind the co-management progress, fishers are unable to exercise the shared powers and this makes the fishers' powers nominal rather than actual. Given that fisheries co-management is in its infancy in Vietnam and is still under development, the findings of this research are useful for government at different levels to build successful co-management systems in Vietnam. It also implies a need for the government to conduct an overview of the co-management system to gain the lessons and experiences from what has been done so far,...