Electric power is a critical infrastructure that is vital to the U.S. economy and national security. Today, the nation's electric power infrastructure is threatened by malicious attacks, accidents, and failures, as well as disruptive natural events. As the electric grid evolves and becomes increasingly interdependent with other critical infrastructures, the nation is challenged to defend against these threats and to advance grid capabilities with reliable defenses. On November 1, 2019, the National Academies of Sciences, Engineering, and Medicine convened a workshop to gather diverse perspectives on current and future threats to the electric power system, activities that the subsector is pursuing to defend itself, and how this work may evolve over the coming decades. This publications summarizes the presentations and discussions from the workshop.