This thesis reviews theoretical and local understandings of sustainability and examines the contributions which the NGO, the Toledo Institute for Development and Environment (TIDE), makes to achieving sustainability in the district of Toledo in Belize, Central America. The study focuses mainly on the Port Honduras Marine Reserve, which TIDE manages in cooperation with the Toledo Fisheries Department. Three sets of theories provide the context: theories of sustainability and sustainable development in general, theories on marine protected areas and fisheries management, and theories on NGOs. Findings show that TIDE has a vague understanding of sustainability and that there is a contradiction between the mission statement which promises a balance between environmental protection and stakeholders' economic needs, and TIDE' s actual strategies for encouraging protection for the Port Honduras Marine Reserve. In addition, TIDE's dysfunctional internal management style, such as lack of communication between the management and staff, spontaneous planning, and frequent staff turn-over prevent the organization from functioning effectively and realizing its goals. TIDE’s approach to sustainability is ineffective because it fails to integrate emic perspectives. TIDE does not take into account that local residents have an their own perspectives on sustainability, which include the fishermen's sense of stewardship for the marine environment. TIDE also does not successfully ensuring a steady income for the fishermen effectively. In light of the theories on sustainability, findings show that most theories are developed in a top-down manner that fail to integrate local understandings of sustainability. The only possible exception are holistic theories that emphasize bottom-up participation. The main recommendation for TIDE and similar NGOs is to develop a holistic approach to sustainability that includes emic perspectives. Consideration of the environment, the consideration of stakeholders' economic needs, the socio-cultural context, and an administrative political framework and a functional internal management style all need to be integrated into a successful approach to sustainability. / Graduation date: 2001 / Best scan available for figures 2-3.