This chapter focuses on both Croatia and Slovenia are now members of the EU, the region of Piran Bay, has played a significant role in determining the relations between these two states. It combines these factors with data from new literature and 20 semi-structured interviews with experts, local stakeholders and relevant authorities, including new biological and threat data, as well as socio-economic and governance opinion. The reflected earlier opinions expressed by Vidmar and Turk who highlighted the absence of marine protected areas (MPAs) for the conservation of large marine vertebrates in Slovenian waters. The ecologically or biologically significant areas (EBSAs) were originally driven by the need to develop MPAs in areas beyond national jurisdictions although the remit has broadened to include marine spatial planning (MSP) also within national jurisdictions. Legal pressure can be applied through the European Court of Justice, and economic incentives can be provided through various programmes to encourage the member states to comply.