Arrivals by international tourists have nearly doubled in the first two decades of the 21st century. Some urban areas have not been well-equipped to deal with the economic, social, and cultural effects of this process, such as overcrowding and tourism gentrification that leads to the transformation of residential neighbourhoods. This paper follows a two-fold aim: (i) to examine the relationship between urban planning and tourism development in three World Heritage Sites along the Mediterranean Sea, Kotor (in Montenegro), Dubrovnik (in Croatia) and Venice (in Italy), which face issues of overcrowding; (ii) to identify the origins of issues that tourism is imposing on urban functions with respect to each case, in order to set the basis of targets in need and to a propose practical framework for tailored policies open to the needs of the local community. The findings show that although the tourism industry plays a key role in the economy of all cases, it imposes significant externalities at the local level.