In Parts 1 and 2 of our Pulse series, we explored the rising demand for electricity and the evolving mix of energy sources powering the U.S. economy. But generating electricity is only part of the equation. To keep the lights on, we need a robust, resilient, and modern transmission system capable of delivering that power where and when it’s needed. In this final installment, we examine the U.S. power grid—how it works, where it’s vulnerable, and what’s being done to strengthen it.