This authors of this report aims to comprehensively assess the spectrum of flexibility resources and options available to help enable increased penetration of variable generation in power system. They lay a framework in which to understand the flexibility requirements necessary to make the transition to high-penetration variable-generation power systems. They then provides a series of fact sheets covering a wide range of options for increasing system flexibility, ranging from flexibility in fossil generation to active power control of renewables, demand management, energy storage options, and market and network options and more—covering 16 potential sources of flexibility in all. Each fact sheet contains an overview of the costs, technical constrains, barriers to deployment and potential role. Though the costs come from a European context, the fact sheets provide a framework for decision makers in weighing the costs and benefits of different flexibility options.