The widespread uptake of renewable power generation is creating new variability and system reliability issues for operators to manage. Power markets are addressing the challenge in different ways, from ramping gas or coal fleets up and down, to enhancing transmission interconnections, leveraging pumped hydro or batteries storage and innovating demand-side management. Policies and market mechanisms are poised to evolve to reward flexible power supply, particularly for zero-carbon solutions. But what are the risks? What changes need to be made to create commercial opportunities for flexibility suppliers? In a transforming energy system, what are the sources of future flexibility?