The objective of this study is to comprehensively assess flexibility needs and solutions in the power systems of the Energy Community Contracting Parties, and to propose solutions that will facilitate renewable energy sources integration and transition to the carbon-neutral energy sector in different scenarios for the Energy Community Contracting Parties in a cost-efficient and secure way. The term “flexibility” in this context means the ability of all power system resources/facilities, including generation, storage, interconnectors, and demand-response, to adjust the electricity output and consumption to maintain nominal frequency at all times.