Subject of this research are the geostrategic features of the Adriatic Sea as an essential part of the syntagm "Adriatic Sea – a link between the continents". The Adriatic Sea is an area where different political and economic interests overlap. Geographic basis and interaction of different socio-economic processes (migrations, political-geographical changes, social and economic transition, littoralization etc.) in the Adriatic region have a key importance in the military and strategic planning. The research has been conducted through several areas, namely: the historical role of the Adriatic Sea and the natural-geographic and socio-economic potentials in the function of military-scale planning as well as their synergistic influence on military and national power. The demographic trends of the Adriatic Croatia were particularly discussed in reference to migrations including the migration crisis. In the context of military analysis, it has to be taken into account that the Adriatic Sea is very narrow and therefore the execution of military and other security operations should be adapted to the littoral space. Furthermore, ecological threats of the Adriatic Sea were analyzed in regard to the eco-protection model at the military training area Rt Kamenjak during military exercises. A special area of the research refers to contemporary geopolitical and geostrategic features of the Adriatic Sea through the analysis of the interdependence of the Adriatic Sea with the central and southeastern European area and the Mediterranean. The purpose of this research is to determine the geostrategic features of the Adriatic Sea, which are, among other things, based on the military-strategic aspect of evaluating natural and social resources of the Adriatic Croatia. In this respect, the research has shown that the distinctive features of the tangent coastal-island-sea area affect planning and implementation of military operations in the littoral karst area. Following the same idea, economic potentials and opportunities for sustainable development and the contemporary ecological threats are presented in the context of military-strategic planning. Present geostrategic features of the Adriatic Croatia are highlighted in the context of the Mediterranean and Central and South-Eastern Europe. During the doctoral thesis work several methods of scientific research were used - methods of modeling, historical methods and methods of analysis and synthesis. Analyzing the geographical basis, frameworks of military-strategic planning have been defined, especially in the context of the importance of the Croatian part of the Adriatic Sea as a strategically important area and resource on which a significant part of the national security is based. An interdisciplinary approach was applied, primarily related to geography, (military) maritime geography, military strategy, oceanography, history, economics, law and political science in order to gain a more complete picture of the geostrategic significance of the Croatian part of the Adriatic Sea. Individual objectives are derived from the main purpose of research, namely the observation and determination of the contemporary geostrategic features of the Adriatic Croatia. Those objectives are inspected more closely in individual chapters of this paper. However, the general aim is to overview and define the contemporary geostrategic features of the Croatian part of the Adriatic Sea through the historical framework and contemporary global and regional geopolitical relations. Taking into account the historical background, Croatia has to maintain and develop its own security and defense system and a type of Armed Forces, which can respond to its unfavorable geostrategic and physical-geographical features. Therefore, defense strategy has to take into account all geographical features of the region as well as its surroundings and structure and equip its Armed Forces in accordance with them, and this way adapting to the requirements of the Theatre of War. Current geostrategic features of the Croatian part of the Adriatic Sea as a potentially most important multiplier of national power requires a permanent analysis of the countenance of modern threats, marine power, capabilities to protect interests and finally their influence on national security. The thesis provides a synthesis of temporary geostrategic features of the Croatian part of the Adriatic Sea with argumentative confirmation of the hypothesis about the geostrategic importance of the Adriatic region for the Croatian state. With the analysis of the geographical features and political-geographical relations, a frame and foundation has been shaped for further military-strategic considerations. The thesis indicates the importance of preservation of potentials and restrictions of the Adriatic region and fosters research and detection of new research problems.