As renewable generation becomes increasingly widespread in power markets globally, the associated variability and impact on system reliability are highlighting the importance of flexible power sources. Today, 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 incentivize and reward flexible power supply, particularly those zero-carbon solutions that can advance energy transition. Is there an optimal balance between variable renewable capacity and flexible power resources, theoretically or in practice? Which flexible solution has the most positive cost curve outlook and at what scale? What are the main policies and market mechanisms addressing flexibility today? What changes need to be made to create commercial opportunities for flexibility suppliers?