Long-term transmission planning over large power systems, such as the European transmission network, is a very large and complex problem. Indeed, transmission planning is usually performed for time horizons comprised between 10 and 20 years and current methods have not coped so far with planning over large areas and with increased uncertainty. This is even more the case when integrating significant shares of renewable generation. Today, any national transmission system in Europe is planned using expert judgement. Therefore, Work Package 8 of the e-Highway2050 project aims at defining a new methodology where longterm planning is formalised as an optimisation problem, and at specifying new tools. Advanced optimisation and simulation methods are investigated to tackle this complex highly combinatorial problem, taking into account the stochastic and dynamic behaviours of the system. A Monte-Carlo approach is used to consider the stochastic dimension of the problem. The problem is solved from a transmission system operator (TSO) perspective, with no control over generation planning. Though, several possible evolutions of the electricity sector are investigated for different scenarios. The main challenges arise from the spatial, temporal and stochastic complexities. Thus, methods to reduce the size of the grid or to choose relevant snapshots among the 8760 hours of a year have been developed.