This publication—which is rooted in seven case studies comprising 15 countries—gauges the economic significance of the integration impacts of variable renewable energy; highlights the need for a system-wide approach to integrating high shares of variable renewable energy; and recommends how to achieve a cost-effective transformation of the power system. It summarises the results of the third phase of the IEA’s Grid Integration of Variable Renewables (GIVAR) project. In addition, it assesses the four available flexible resources (generation, grid infrastructure, storage and demand side integration) in terms of their technical performance and cost-effectiveness.