This research assessed the sustainability of protected area-based tourism systems in Nepal. The research was composed of three interrelated studies. The first study evaluated different approaches to protected area governance. This was a multiple-case study research involving three protected areas in Nepal: the Annapurna Conservation Area, Chitwan National Park, and the Kanchenjunga Conservation Area. Data were collected from various published and unpublished sources and supplemented with 55 face-to-face interviews. Results revealed that outcomes pertaining to biodiversity conservation, community livelihoods, and sustainable tourism vary across these protected areas. The study concluded that there is no institutional panacea for managing protected areas. The second study diagnosed the sustainability of tourism in two destination communities: Ghandruk and Sauraha, which are located within the Annapurna Conservation Area and Chitwan National Park, respectively. A systemic, holistic approach--the social-ecological system framework--was used to analyze the structures, processes, and outcomes of tourism development. Data collection involved 45 face-to-face semi-structured interviews and a review of published and unpublished documents. Results revealed that tourism has several positive and a few negative sociocultural, economic, and ecological outcomes in both communities. Overall, tourism has progressed towards sustainability in these destinations. The third study examined tourism stakeholders' perspectives regarding sustainable tourism outcomes in protected areas. The study compared the responses of residents with residents, as well as tourists with tourists, across the Annapurna Conservation Area and Chitwan National Park. Tourism sustainability was evaluated with six tourism impact subscales measuring negative and positive ecological, economic, and social impacts. Data were collected using the survey method. Respondents included 230 residents and 205 tourists in Annapurna, and 220 residents and 210 tourists in Chitwan. The findings revealed that the residents across these protected areas perceived positive and negative impacts differently, as did the tourists, suggesting that the form of tourism development affects the sustainability outcomes in protected areas. Overall, this research concluded that protected areas and tourism are intricately related, and sustainable management of a protected area-based tourism system requires a polycentric adaptive approach that warrants a broad participation of relevant stakeholders.