Power systems are designed and operated to efficiently manage variability and uncertainty in electricity demand and resource availability. Variable renewable energy (VRE) increases this inherent variability and uncertainty, and thus increases the need for flexibility. Systems with significant variability and uncertainty require flexible generators that can rapidly change output, operate efficiently at lower outputs, and operate for short durations. This flexibility in turn can (1) reduce the need to curtail (decrease the output of) solar and wind generation, (2) improve investor confidence in VRE and revenue streams, (3) decrease the risk of negative market pricing (which results when conventional generators cannot turn down to low outputs during times of high VRE output) and (4) reduce environmental impacts by increasing system efficiency and maximizing the utilization of VRE. This fact sheet addresses sources of flexibility and includes definitions of related terms.