The thesis defines the concept of mass tourism and its impact on the destinations Croatia and Spain, but also on the environment and its natural balance. The aim of the research is to analyse and elaborate positive and negative aspects of mass tourism development on destinations and make comparison of these two destinations. Furthermore, the goal is to analyze the existing possibilities of tourism development, sustainable development strategies and to define guidelines for future development. The thesis underlines the importance of the implementation of sustainable tourism development policies in order to prevent or at least mitigate the negative effects that it can yield. For the purpose of the thesis secondary data from the university books, scientific and professional articles and statistical data and publications from the various internet data are used. Croatia and Spain are developing its economy mostly based on tourism, and it is significant source of income. Both destinations are trying to extend the tourist season, reduce the seasonal mass beach tourism, although Spain has more success than Croatia. During recent years there has been more developed awareness among all stakeholders of the tourist flow on the environmental, cultural and historical resources of destinations and is increasingly encourage the development of green business in the tourism and hospitality award of grants, encourages the production of local indigenous products and the development of traditional events. Sustainable development is more present in all aspects of human life and it raises the awareness of protecting their own environment. Thus the tourist who goes to a destination looks after the environment at their destination. Local and regional authorities raise awareness on the conservation of heritage and culture through various programs in which they involve the local community.