The Role of the Souther Corridor Region in the development of the EU Gas Infrastructure and the quest for diversification of supply sources and routes The capacity and integration of gas transmission networks into a common European network generally depends on the historical development of supply and sources. The development of new gas infrastructure supports the three pillars of the European energy policy: market integration, security of supply and sustainability. Ultimately, it enables and facilitates a liquid and competitive common gas market, through increased market participation and integration. The rationale behind the key European gas transmission projects is increasing the flexibility and integration of energy markets by ensuring different connections, more alternatives of supply sources and at the same time increasing the cross-border capacities. Despite the fact that a very significant part of the natural gas, which is used in the EU, crosses at least one border, the flexibility of its transmission system still needs to be increased. The resulting increased flexibility of the European gas system will enable and enhance supply diversification thus improving the security of gas supply. The integration level of different gas transmission networks is also dictated by the characteristics of larger projects in which the EU member countries are included. Approximately half of these projects are intended to increase the existing capacities and the other half to develop new gas transmission infrastructure with new capacities. Gas infrastructure can also have a significant role to play in improving sustainability in Europe, since natural gas is expected to have a key role in helping the EU meet its environmental targets as the cleanest available fossil fuel and the one better suited to complement the intermittency of most renewable energy sources used for power generation.