ENThe legal concept of energy security according to the doctrine, national and European Union legislation is analysed in this thesis. The concept of the energy security of the Republic of Lithuania is discussed through the assessment of the key threats to the energy system defined in the national legislation and of the measures, by which the goal of ensuring energy security is declared. Considering the measures applied under the national legislation in order to ensure energy security, the compliance of the applicable measures with the national concept of energy security established in the European Union and international legal acts is analysed, the key problems related with the application of the latter measures are identified.