Sustainable development of tourism occurs as a result of the growing number of tourists and pressure on the destination, its resources and the need to preserve natural values and cultural heritage so that they can be valorised in the long run. Such development aims to reduce the negative impacts of tourism on the environment and society and increase the positive effects on the economy. Sustainable tourism development implies ecological, socio-cultural and economic sustainability. Mass tourism is becoming an increasing problem in tourist destinations, and Portugal is an example of a destination in which tourism began to develop rapidly and has reached just the stage of mass. High concentration of tourists in the most visited tourist destinations in Portugal often surpass the number of local population, which is one of the main characteristics of mass tourism. Such tourism development is not sustainable in the long run. In order to further development of tourism in future, but in the same time to reduce the negative impacts of its development, Portugal adopts „Tourism Strategy in the period 2021-2017”. In this document Portugal emphasizes the importance of adopting the principles of sustainable tourism development.