This document, produced by the European Network of Transmission System Operators for Gas (ENTSOG), is the fourth edition of the pan-European Ten Year Net- work Development Plan (TYNDP). To comply with the Cost-Benefit Analysis (CBA) requirement of the TEN-E Regulation, this TYNDP covers an extended time period ranging from 2015 to 2035 and provides a wide ranging view of how European gas infrastructure and supply ad- equacy could develop over the next two decades. The regulatory requirement on ENTSOG to publish the Union-wide TYNDP every two years stems from the 3 rd Energy Package. The original aims were to identify possible investment gaps and to assess the evolution of the supply adequacy. With the entry into force of the TEN-E Regulation in May 2013, the TYNDP has acquired a new dimension as it is now the first step of the Project of Common Interest (PCI) process. Every PCI candidate must submit its project to ENTSOG for inclusion in TYNDP. ENTSOG will then apply the Cost-Benefit Analysis (CBA) methodology, which has been developed for this TYNDP. In order to ensure the consistency of the TYNDP and the CBA methodology, ENTSOG has merged the two consultation processes. The main objectives were the adapta- tion of the methodology, first published in November 2013, and the definition of the associated data set. It also offered the opportunity to run a case-study on a sample of projects. This consultation process represented a key step in the preparation of the PCI assessment by ENTSOG as it was supposed to gather the knowledge of all stakeholders. This is of particular importance for data related to supply and price scenarios which are beyond ENTSOG remit. The development and maintenance of gas infrastructure supports the three pillars of the European energy policy: security of supply, competition and sustainability. It facilitates a liquid and hence a competitive internal gas market by increasing physical market integration. The resulting flexibility of the European gas system will enable and enhance supply diversification and Security of Supply, even in the case of declining indigenous production. Gas infrastructure will also play an important role in improving sustainability in the EU by helping to meet its environmental targets.