The electric power industry is experiencing a significant shift that requires more integrated grid planning practices. Utilities previously created generation, transmission, distribution, and customer program plans separately, each optimized on its own. Today, factors like rapid decarbonization, the growth of distributed energy resources (DERs), and the electrification of transportation and heating are revealing the flaws of this isolated approach. Planners and regulators are increasingly advocating for integrated system planning—a coordinated method combining resource planning, transmission–distribution coordination, and customer-side initiatives—to ensure the grid of the future is both reliable and affordable.