Energy security as a part of national security has been given a lot of attention in the literature. There are a lot of definitions of energy security, but all of them include the famous "triad": security of supply, affordability and sustainability in regards to energy. These three aspects are also the ones that the EU energy security strategy has at its forefront. However, research has consistently shown that achieving these three goals at the same time is impossible due to different internal and external, economic and political factors. The past sixty years of the European energy policy has clearly proven the ambiguity of the EU energy security strategy. Recent developments in the field, however, have led to a renewed focus on the security of supply.This book will examine the changing nature of the strategy and evaluate its progress. The objectives of this research are to determine whether the development of the European gas market liberalization is to be beneficial for the security of supply.