Background: The aim of this study is to present the current types of energy storage systems (ESS), analyze and discuss the legislative and regulatory regime of ESS within the EU, focusing on the Greek case, and finally, make recommendations about how policymakers can respond to the rapid growth in emerging energy storage solutions and give incentives to stakeholders in order to promote the integration of energy storage technologies into the grid. Methods: The method and the data sources used for the present thesis consist of qualitative tools, i.e., systematic literature review, document analysis, as well as analysis of the legislative and regulatory regime concerning energy storage systems within the EU. This thesis used publicly available data sources, especially legal sources from the EU and Greek legal framework. Conclusion: The current study provides a useful insight into the ESS technologies, which constitute a linchpin in the transformation of the power system within the EU and in the pathway towards a decarbonized energy sector and focus on the legal and regulatory regime within the EU and especially on the emerging Greek legislative framework.