Electricity is an integral part of all modern economies, supporting a range of critical services from healthcare to banking to transportation. The secure supply of electricity is thus of paramount importance. The power sector is going through fundamental changes: decarbonisation with fast growth in variable renewable sources, digitalisation expanding the surface for cyberattacks, and climate change leading to more extreme weather events. In response, governments, industries and other stakeholders will need to improve their frameworks for ensuring electricity security through updated policies, regulations and market designs. This report surveys the ongoing multiple transformations in the electricity sector, which are leading to a new system in the future. For the first time, three key aspects of electricity security are addressed in one report: energy transitions with more variable renewables, cyber risks, and climate impacts. In addition, the roles of new technologies and demand-side response, and electrification of other sectors are explored. Examples and case studies of all these changes are taken from power systems around the world. Existing frameworks that value and provide electricity security are described, and best practices offered along with recommendations to guide policy makers as they adjust to the various trends underway.