In this Master’s thesis, we explain the Significance of Energy to our Society, as well as the connection among the concepts of Cybersecurity, Energy and Energy Security. In addition, we present various Cybersecurity related Definitions and the Most Important Cybersecurity Threats, as well as list the Most Significant Energy Cybersecurity Incidents Worldwide. Moreover, we present the Authorities which practice and the Institutions which exercise Cybersecurity in Greece and compare them with those of EU countries with similar Domestic Energy Production Mix to Greece (i.e. with Italy, Spain, Portugal and Germany) and with USA. Furthermore, we review the great variety of the Critical Energy Infrastructures in Greece with examples and analyse the Greek Energy Cybersecurity Laws. Finally, we present Facets of Reaction to a Cyber-attack in Greece and in USA – basis the information available, along with the EU Regulations and Directives related to Energy Cybersecurity adopted since 2019, and their enforcement into the Greek Legal System.