Scope: This guide provides engineering principles, practices, and guidelines for the selection and application of fast response systems of customer-side loads in modern power grids. It considers the impact of increasing the penetration of renewable energy sources in power systems. The definitions, classification and characteristics of rapid load response resources are elaborated in this guide, such as active response loads, interruptible loads and energy storage. It provides complementary approaches for mitigating frequency stability issues of bulk power systems and microgrids, as well as absorbing renewable power. Function architecture, control strategy, and system implementation for quick response systems of customer-side loads are explored in this guide.