The main purpose of this paper is a comprehensive investigation of the Greek electricity production and transmission system with a focus on the integration of renewable energy sources. A techno-economic model of the Continental European electricity sector called ATLANTIS is used to analyse the development of the Greek power system. Several different projections—including national, ENTSO-E and European scenarios—have been investigated and the officially published national development strategy of Greece until 2027 has been chosen for the simulation. Starting with the modelling of the technical system, that considers a pre-defined development path for the power plant park and the transmission grid, several techno-economic investigations are performed. These include the effects of an increasing electricity demand on the electricity production with respect to several technologies, the imports, the load flows and the CO2 emissions. Additionally, the age structure and the value of the power plant park are analysed. The results illustrate a remarkable shift from a fossil fuel dominated electricity structure to an electricity system with a high share of renewable energies. The integration of the wind power and solar/PV production capacities requires a corresponding enhancement of the Greek transmission grid including several new cross-border connections to the neighbouring countries. Therefore, all important transmission grid projects based on the TYNDP and the national grid operator have been included in the physical transmission system, which covers long distance connections between the islands and the mainland as well as connections between the islands themselves by using also the HVDC technology. Furthermore, it is shown that due to the reduced lifetime of the RES technologies compared to the fossil-fuelled and the hydro power plants the up-coming re-investment cycles have to be planned carefully. Finally, the paper closes with a comprehensive discussion of the outcomes and gives an outlook of the ongoing and the future works, which cover e.g. a stronger linkage of the whole Mediterranean electricity systems to the European transmission grid according to a European energy strategy.