This report is labeled “Traffic Light Protocol (TLP) WHITE,”1 a designation meaning that recipients may share this report freely without restriction. NERC conducted its fourth biennial (once every two years) grid security and emergency response exercise, GridEx IV, from November 15–16, 2017. With 6,500 individuals and 450 organizations participating across industry, law enforcement, and government agencies, GridEx IV consisted of a two-day distributed play exercise and a separate executive tabletop on the second day. The exercise provided an opportunity for various stakeholders in the electricity sector to respond to simulated cyber and physical attacks that affect the reliable operation of the grid, fulfilling NERC’s mission to assure the effective and efficient reduction of risks to the reliability and security of the BPS. Led by NERC’s E-ISAC, GridEx IV was the largest geographically distributed grid security exercise to date. Electric utilities continue to use the planning materials for separate exercises with NERC, government, and consultant support.