Wind power and solar photovoltaics (PV) are crucial to meeting future energy needs while decarbonising the power sector. Deployment of both technologies has expanded rapidly in recent years, one of the few bright spots in an otherwise bleak picture of clean energy progress. However, the inherent variability of wind power and solar PV raises unique and pressing questions. Can power systems remain reliable and cost-effective while supporting high shares of variable renewable energy (VRE)? And if so, how?Based on a thorough review of the integration challenge, this publication gauges the economic significance of VRE integration impacts, highlights the need for a system-wide approach to integrating high shares of VRE and recommends how to achieve a cost-effective transformation of the power system.This book summarises the results of the third phase of the Grid Integration of VRE (GIVAR) project, undertaken by the IEA over the past two years. It is rooted in a set of seven case studies, comprising 15 countries on four continents. It deepens the technical analysis of previous IEA work and lays out an analytical framework for understanding the economics of VRE integration impacts. Based on detailed modelling, the impact of high shares of VRE on total system costs is analysed. In addition, the four flexible resources which are available to facilitate VRE integration – generation, grid infrastructure, storage and demand side integration – are assessed in terms of their technical performance and cost-effectiveness.