Increased use of renewable energy sources and the creation of an internal electricity market have resulted in higher and more volatile power flows in the transmission grid. Due to a sustained climate policy, the share of renewable energy sources in electricity generation will increase over the coming decades. To cope with the expected future power flows, new transmission system investments are inevitable. Over the coming decades, in Europe alone, transmission system investments worth several hundred billion Euro are foreseen.The aim of transmission system planning is to provide an expansion scheme for the transmission network in order to fulfil future grid requirements. Transmission system planning is a complex and multidimensional problem due to the high number of uncertainties and influencing parameters. Therefore, it is difficult to obtain a unique optimal investment plan which satisfies all technical, economical, social and environmental constraints. Many different scenarios regarding generation, demand, market prices, technological and social developments have to be analysed in the planning process in order to assess uncertainties and minimize the investment risks. This dissertation provides the building blocks of a transmission system investment optimization methodology to assess different future scenarios in a fast and effective way. The methodology consists of different modules which can be combined to a single framework or used separately by transmission system planners. The optimization takes the geographic and demographic properties of the area of interest into account which are not included in existing transmission system investment optimization methodologies. It delivers a stepwise transmission system investment plan containing the optimal time point, power rating, transmission route and transmission technology for new investments. Another new feature of the developed methodology is the determination of strong grid nodes based on probabilistic optimal power flow solutions.