This report summarises findings on wind integration from the 17 countries orsponsors participating in the International Energy Agency Wind TechnologyCollaboration Programme (IEA Wind TCP) Task 25 from 2006–2020. Both realexperience and studies are reported. Many wind integration studies incorporatesolar energy, and most of the results discussed here are valid for other variablerenewables in addition to wind.The national case studies address several impacts of wind power on electricpower systems. In this report, they are grouped under long-term planning issuesand short-term operational impacts. Long-term planning issues include gridplanning and capacity adequacy. Short-term operational impacts includereliability, stability, reserves, and maximising the value of wind in operationaltimescales (balancing related issues). The first section presents the variability anduncertainty of power system-wide wind power, and the last section presentsrecent studies toward 100% shares of renewables. The appendix provides asummary of ongoing research in the national projects contributing to Task 25 for2021–2024.The design and operation of power and energy systems is an evolving field. Asambitious targets toward net-zero carbon energy systems are announcedglobally, many scenarios are being made regarding how to reach these futuredecarbonized energy systems, most of them involving large amounts of variablerenewables, mainly wind and solar energy. The secure operation of powersystems is increasingly challenging, and the impacts of variable renewables, newelectrification loads together with increased distribution system resources willlead to somewhat different challenges for different systems. Tools and methodsto study future power and energy systems also need to evolve, and both shorttermoperational aspects (such as power system stability) and long-term aspects(such as resource adequacy) will probably see new paradigms of operation anddesign. The experience of operating and planning systems with large amounts ofvariable generation is accumulating, and research to tackle the challenges ofinverter-based, nonsynchronous generation is on the way. Energy transition anddigitalization also bring new flexibility opportunities, both short and long term.