This paper presents the problem regarding sustainable development and ecotourism in protected natural areas, especially in Romania, but also internationally. In the first part of the work are presented the specific features and principles of ecotourism. The durable development concept is now a presence in all economical, social domains, inclusive in the tourism field. Any form of tourism should respect the durable development principles: from ecotourism, rural tourism, cultural tourism, business tourism, congresses or automotive tourism. Starting from the premise that the ecotourism activity develops, with priority in the natural areas, in the next part of the paper we have made a brief history of ecotourism internationally and in Romania and classified the main protected areas in Romania. The last part of the paper is a case study which shows a parallel between two renowned national parks, Yellowstone National Park in U.S.A. and Retezat National Park in Romania. The purpose of this article is also to promote ecotourism as the main form of tourism that capitalizes on the specifics of protected natural areas in Romania, for example in the Retezat National Park.