Like spatial and time scales, system quality is also a key component in achieving STD. In this paper, system quality means ‘the state of health in a tourist destination, sustaining the benefits of local community, satisfaction of tourist experience, and conservation of natural resources’. System quality involves judgments about the functionality/health of the tourism destination. Thus, where a destination meets the requirements of the three STD components mentioned above, the system quality of the destination would be necessarily high. A tourist destination means ‘a tourist attraction (human made or natural), including the human system and the ecosystem, influenced by tourism activities’. Therefore, to become a sustainable tourist destination, the two systems must be sustainable simultaneously. This is one of the most important assumptions in the assessment of tourism sustainability in this study. The Egg of Sustainability of Prescott-Allen (see IUCN, 1995, pp. 154–155) can be modified and applied to the context of tourism. Human societies form a sub-system within the ecosystem, just as the yolk of an egg is within the white. For an egg to be good and edible, both the yolk and the white have to be good. Similarly, a tourist destination is sustainable only if both the human condition and the condition of the ecosystem are satisfactory or improving. Human beings and ecosystems are seen as equally important. If the condition of either is unsatisfactory or worsening, the destination is unsustainable.