The increase of renewable energy share in the power generation mix to achieve national and international targets of greenhouse gases emissions reduction comes with important consequences, especially for the electricity grid that has to increase its flexibility to assure the quality and reliability of supply. This requirement can be much more relevant when dealing with islands, as they have limited (or no) interconnections to the continent and thus have to rely more on more flexibile options to ensure the secure and cost-efficient operation of their energy system. In this context, a longterm prospective study, based on technico-economic optimization of TIMES model generator, is carried out to explore decarbonisation pathways that ensure grid flexibility of the two investigated European islands-Procida in Italy and Hinnøya in Norway. Emphasis is given to technical, economic and policy aspects of the evolution of the islands' power systems.