Harbours with protruding breakwaters that are built on coasts cause alterations to the wave field, which attacks the coastline adjacent to the structures. As a result, sandy beaches often start eroding. Examples from Greece and Cyprus are presented, veryfying that the causes of the phenomenon, which are (i) the obstruction of longshore sand movement, and (ii) the longshore sand transport from the wave-action zone to the calm wave-shelter zone triggered by wave diffraction, work almost always simultaneously and none can be underestimated. A large discussion is held internationally indicating that although the construction of harbours is meant to enhance tourism and local economy, it very often leads to environmental and socio-economic failure that are analysed in the present paper. In addition, the paper discusses the methods of beach protection in such cases and gives guidelines for following effective practices in Greece.