A large number of countries in the world generate the greatest income through tourism, which is considered to be the fastest growing industry in the world. Tourism is oriented towards working with people, but like other forms of development it can lead to various problems. One of them is environmental pollution and more responsible, alternative forms of tourism are being approached. One of them is ecotourism, which focuses on the stay of tourists in nature, environmental protection and the development of environmental awareness. The Republic of Croatia is rich in natural beauty and as such is recognized as one of the most desirable tourist destinations in the world. Which leads to the fact that tourism as a branch of the economy is the most developed sector in the Republic of Croatia. Among the many natural beauties are Plitvice Lakes, which were placed under the protection of UNESCO, back in 1979. Plitvice Lakes is the oldest and largest national park in the Republic of Croatia, located in Lika. They are spread over an area of almost 30,000 ha and are rich in flora and fauna. They consist of 16 lakes, divided into Upper Lakes (of which there are 12), and Lower Lakes (4 of them), which are determined by natural barriers. Plitvice Lakes record an increase in tourist visitors from year to year, which leads to congestion and pollution of the area, due to overcrowding and crowds that are created in the national park, especially during the summer months. Here we come to the problem called - sustainable development, which becomes an urgent need for the ecosystem in which the Plitvice Lakes National Park is located. Sustainable development is based on four main principles: environmental sustainability, economic sustainability, technological sustainability and socio-cultural sustainability. The paper will present the sustainability of ecotourism on the example of the mentioned national park.