Eight Transmission System Operators (BE, CZ, FR, DE, IT, PT, CH, PL) and ENTSO-E, together with 4 associations of technology manufacturers, and 16 RTD performers propose a 3-year R&D project to develop and to apply a methodology for the long-term development of the Pan-European transmission network. The project aims at delivering a top-down methodology to support the planning from 2020 to 2050. First, it implements a set of future power scenarios, including generation units, the possible use of electricity storage and demand-side management solutions: scenarios for power localization are proposed with assumptions on the energy mix in each of the connected clusters covering the ENTSO-E area. Network studies are performed to detect the weak points when implementing the scenarios for 2050. Grid architectures options and a modular development plan are then proposed, including electricity highways, on the basis of power flow calculations, network stability analysis, socio-economic, network governance considerations, and with remarks from the consultation of European stakeholders. In parallel, an advanced planning methodology is designed, developed and tested with academic laboratories to address a few critical aspects of the above planning methodology, which may impact the robustness of the resulting architectures. This enhanced approach takes into account the correlated uncertainties in renewable generation and consumption, potential voltage and stability issues, and black-out risks including the feasibility of defence plans to avoid uncontrolled cascading failures of the candidate architectures. It includes the use of non-linear detailed models of power grids and stochastic optimization techniques. The dissemination is coordinated by ENTSO-E to reach the widest audience and to prepare the exploitation of the results. Standardization and complementary research efforts are pointed out for the future investment optimization with the support of the manufacturing industry.