The North American Electric Reliability Corporation (NERC) conducted its third biennial grid security and emergency response exercise, GridEx III, on November 18-19, 2015. NERC’s mission is to assure the reliability of the bulk power system (BPS) and GridEx III provided an opportunity for industry and other stakeholders to respond to simulated cyber and physical attacks affecting the reliable operation of the grid. Led by NERC’s Electricity Information Sharing and Analysis Center (E-ISAC), GridEx III was the largest geographically distributed grid security exercise to date. GridEx III consisted of a two-day distributed play exercise and a separate executive tabletop on the second day. More than 4,400 individuals from 364 organizations across North America participated in GridEx III, including industry, law enforcement, and government agencies.